diff --git a/boulder_base.libraries.yml b/boulder_base.libraries.yml
index 85a65c93..e0f66abf 100644
--- a/boulder_base.libraries.yml
+++ b/boulder_base.libraries.yml
@@ -570,3 +570,11 @@ ucb-trusted-content-preview:
   css:
     theme:
       css/block/ucb-trusted-content-block.css : {weight: 5}
+
+ucb-tos-acceptance:
+  version: 1.x
+  js:
+    js/ucb-tos-acceptance.js: { }
+  css:
+    theme:
+      css/ucb-tos-acceptance.css: { weight: 5 }
diff --git a/css/ucb-tos-acceptance.css b/css/ucb-tos-acceptance.css
new file mode 100644
index 00000000..3fd31c57
--- /dev/null
+++ b/css/ucb-tos-acceptance.css
@@ -0,0 +1,133 @@
+/* TOS Acceptance Modal Styles */
+.ucb-tos-modal {
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+  z-index: 10000;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  opacity: 0;
+  visibility: hidden;
+  transition: opacity 0.3s ease, visibility 0.3s ease;
+}
+
+.ucb-tos-modal[hidden] {
+  display: none;
+}
+
+.ucb-tos-modal.ucb-tos-modal-visible {
+  opacity: 1;
+  visibility: visible;
+}
+
+.ucb-tos-modal-backdrop {
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+  background-color: rgba(0, 0, 0, 0.5);
+  z-index: 1;
+}
+
+.ucb-tos-modal-dialog {
+  position: relative;
+  z-index: 2;
+  width: 90%;
+  max-width: 600px;
+  max-height: 90vh;
+  overflow-y: auto;
+  background: #fff;
+  border-radius: 4px;
+  box-shadow: 0 4px 20px rgba(0, 0, 0, 0.3);
+  transform: scale(0.9);
+  transition: transform 0.3s ease;
+}
+
+.ucb-tos-modal.ucb-tos-modal-visible .ucb-tos-modal-dialog {
+  transform: scale(1);
+}
+
+.ucb-tos-modal-content {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+}
+
+.ucb-tos-modal-header {
+  padding: 1.5rem;
+  border-bottom: 1px solid #e0e0e0;
+}
+
+.ucb-tos-modal-title {
+  margin: 0;
+  font-size: 1.5rem;
+  font-weight: 600;
+}
+
+.ucb-tos-modal-body {
+  padding: 1.5rem;
+  flex: 1;
+  overflow-y: auto;
+}
+
+.ucb-tos-modal-body p {
+  margin-bottom: 1rem;
+}
+
+.ucb-tos-modal-body a {
+  color: #005fcc;
+  text-decoration: underline;
+}
+
+.ucb-tos-modal-body a:hover,
+.ucb-tos-modal-body a:focus {
+  color: #004499;
+}
+
+.ucb-tos-modal-footer {
+  padding: 1.5rem;
+  border-top: 1px solid #e0e0e0;
+  display: flex;
+  justify-content: flex-end;
+  gap: 1rem;
+}
+
+.ucb-tos-accept-button {
+  min-width: 120px;
+}
+
+.ucb-tos-accept-button:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+/* Hide aria-live region visually but keep it accessible to screen readers */
+.ucb-tos-modal-live-region {
+  position: absolute;
+  left: -10000px;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+}
+
+/* Responsive adjustments */
+@media (max-width: 768px) {
+  .ucb-tos-modal-dialog {
+    width: 95%;
+    max-height: 95vh;
+  }
+
+  .ucb-tos-modal-header,
+  .ucb-tos-modal-body,
+  .ucb-tos-modal-footer {
+    padding: 1rem;
+  }
+
+  .ucb-tos-modal-title {
+    font-size: 1.25rem;
+  }
+}
diff --git a/js/ucb-tos-acceptance.js b/js/ucb-tos-acceptance.js
new file mode 100644
index 00000000..65c6b37b
--- /dev/null
+++ b/js/ucb-tos-acceptance.js
@@ -0,0 +1,273 @@
+/**
+ * @file
+ * TOS Acceptance modal behaviors.
+ */
+(function (Drupal) {
+  'use strict';
+
+  // Store original body overflow style to restore later.
+  let originalBodyOverflow = '';
+
+  Drupal.behaviors.ucbTosAcceptance = {
+    attach: function (context, settings) {
+      // Check if we should show the TOS modal.
+      const tosData = settings.ucb_tos_acceptance;
+      if (!tosData || !tosData.show_modal) {
+        return;
+      }
+
+      // Create modal if it doesn't exist.
+      let modal = document.querySelector('.ucb-tos-modal');
+      if (!modal) {
+        modal = createTosModal(tosData.tos_url);
+        document.body.appendChild(modal);
+        
+        // Attach event listeners only once when modal is created.
+        attachEventListeners(modal);
+      }
+
+      // Show the modal.
+      showModal(modal);
+    }
+  };
+
+  /**
+   * Attaches event listeners to the modal.
+   */
+  function attachEventListeners(modal) {
+    const dialog = modal.querySelector('.ucb-tos-modal-dialog');
+    const acceptButton = modal.querySelector('.ucb-tos-accept-button');
+    
+    // Handle accept button.
+    if (acceptButton) {
+      acceptButton.addEventListener('click', function(e) {
+        e.preventDefault();
+        handleAcceptance(acceptButton, modal);
+      });
+    }
+
+    // Implement focus trap for keyboard navigation.
+    if (dialog) {
+      dialog.addEventListener('keydown', function(e) {
+        handleFocusTrap(e, dialog);
+      });
+    }
+
+    // Note: Close button, backdrop click, and Escape key handlers removed
+    // to force users to accept TOS before continuing.
+  }
+
+  /**
+   * Handles focus trapping within the modal.
+   */
+  function handleFocusTrap(e, dialog) {
+    // Get all focusable elements within the modal.
+    const focusableElements = dialog.querySelectorAll(
+      'a[href], button:not([disabled]), textarea:not([disabled]), input:not([disabled]), select:not([disabled]), [tabindex]:not([tabindex="-1"])'
+    );
+    
+    if (focusableElements.length === 0) {
+      return;
+    }
+
+    const firstElement = focusableElements[0];
+    const lastElement = focusableElements[focusableElements.length - 1];
+
+    // If Tab is pressed and focus is on the last element, loop to first.
+    if (e.key === 'Tab' && !e.shiftKey && document.activeElement === lastElement) {
+      e.preventDefault();
+      firstElement.focus();
+    }
+    // If Shift+Tab is pressed and focus is on the first element, loop to last.
+    else if (e.key === 'Tab' && e.shiftKey && document.activeElement === firstElement) {
+      e.preventDefault();
+      lastElement.focus();
+    }
+  }
+
+  /**
+   * Creates the TOS modal HTML structure.
+   */
+  function createTosModal(tosUrl) {
+    const modal = document.createElement('div');
+    modal.className = 'ucb-tos-modal';
+    modal.setAttribute('role', 'dialog');
+    modal.setAttribute('aria-labelledby', 'ucb-tos-modal-title');
+    modal.setAttribute('aria-describedby', 'ucb-tos-modal-description');
+    modal.setAttribute('aria-modal', 'true');
+    
+    // Create aria-live region for status updates.
+    const liveRegion = document.createElement('div');
+    liveRegion.setAttribute('role', 'status');
+    liveRegion.setAttribute('aria-live', 'polite');
+    liveRegion.setAttribute('aria-atomic', 'true');
+    liveRegion.className = 'ucb-tos-modal-live-region';
+    liveRegion.style.cssText = 'position: absolute; left: -10000px; width: 1px; height: 1px; overflow: hidden;';
+    document.body.appendChild(liveRegion);
+    
+    modal.innerHTML = `
+      <div class="ucb-tos-modal-backdrop"></div>
+      <div class="ucb-tos-modal-dialog">
+        <div class="ucb-tos-modal-content">
+          <div class="ucb-tos-modal-header">
+            <h2 id="ucb-tos-modal-title" class="ucb-tos-modal-title">Terms of Service</h2>
+          </div>
+          <div class="ucb-tos-modal-body" id="ucb-tos-modal-description">
+            <p>Please review and accept our Terms of Service to continue using this site.</p>
+            <p>
+              <a href="${tosUrl}" target="_blank" rel="noopener noreferrer">View Terms of Service</a>
+            </p>
+          </div>
+          <div class="ucb-tos-modal-footer">
+            <button type="button" class="ucb-tos-accept-button btn btn-primary">Accept</button>
+          </div>
+        </div>
+      </div>
+    `;
+    
+    // Store reference to live region on modal for easy access.
+    modal.liveRegion = liveRegion;
+    
+    return modal;
+  }
+
+  /**
+   * Shows the TOS modal.
+   */
+  function showModal(modal) {
+    // Prevent background scrolling.
+    originalBodyOverflow = document.body.style.overflow;
+    document.body.style.overflow = 'hidden';
+    
+    modal.classList.add('ucb-tos-modal-visible');
+    modal.removeAttribute('hidden');
+    
+    // Focus the accept button for accessibility.
+    const acceptButton = modal.querySelector('.ucb-tos-accept-button');
+    if (acceptButton) {
+      setTimeout(function() {
+        acceptButton.focus();
+      }, 100);
+    }
+    
+    // Announce modal to screen readers.
+    if (modal.liveRegion) {
+      modal.liveRegion.textContent = 'Terms of Service dialog opened. Please review and accept the Terms of Service to continue.';
+    }
+  }
+
+  /**
+   * Hides the TOS modal.
+   */
+  function hideModal(modal) {
+    // Restore background scrolling.
+    document.body.style.overflow = originalBodyOverflow;
+    
+    modal.classList.remove('ucb-tos-modal-visible');
+    modal.setAttribute('hidden', '');
+    
+    // Clear live region.
+    if (modal.liveRegion) {
+      modal.liveRegion.textContent = '';
+    }
+  }
+
+  /**
+   * Handles TOS acceptance via AJAX.
+   */
+  function handleAcceptance(button, modal) {
+    // Disable button to prevent multiple submissions.
+    button.disabled = true;
+    const originalButtonText = button.textContent;
+    button.textContent = 'Processing...';
+    
+    // Update aria-live region to announce processing state.
+    if (modal.liveRegion) {
+      modal.liveRegion.textContent = 'Processing your acceptance of the Terms of Service. Please wait.';
+    }
+
+    // Get accept URL from settings, fallback to default.
+    const tosData = drupalSettings.ucb_tos_acceptance || {};
+    const acceptUrl = tosData.accept_url || '/ucb-user-invite/tos-acceptance/accept';
+
+    // Get CSRF token and make request.
+    getCsrfToken()
+      .then(function(csrfToken) {
+        // Make AJAX request.
+        return fetch(acceptUrl, {
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+            'X-CSRF-Token': csrfToken,
+          },
+          credentials: 'same-origin',
+        });
+      })
+      .then(function(response) {
+        if (!response.ok) {
+          throw new Error('Network response was not ok');
+        }
+        return response.json();
+      })
+      .then(function(data) {
+        if (data.success) {
+          // Announce success to screen readers.
+          if (modal.liveRegion) {
+            modal.liveRegion.textContent = 'Terms of Service accepted successfully.';
+          }
+          
+          // Hide modal on success.
+          hideModal(modal);
+          
+          // Optionally show a success message via Drupal.announce.
+          if (typeof Drupal !== 'undefined' && Drupal.announce) {
+            Drupal.announce('Terms of Service accepted successfully.', 'polite');
+          }
+        } else {
+          throw new Error(data.message || 'Failed to accept TOS');
+        }
+      })
+      .catch(function(error) {
+        console.error('Error accepting TOS:', error);
+        button.disabled = false;
+        button.textContent = originalButtonText;
+        
+        // Show accessible error message via aria-live region.
+        const errorMessage = 'An error occurred while accepting the Terms of Service. Please try again.';
+        if (modal.liveRegion) {
+          modal.liveRegion.textContent = errorMessage;
+        }
+        
+        // Also use Drupal.announce if available for better screen reader support.
+        if (typeof Drupal !== 'undefined' && Drupal.announce) {
+          Drupal.announce(errorMessage, 'assertive');
+        }
+        
+        // Focus the button again so user can retry.
+        setTimeout(function() {
+          button.focus();
+        }, 100);
+      });
+  }
+
+  /**
+   * Gets the CSRF token from Drupal.
+   */
+  function getCsrfToken() {
+    // Fetch token from Drupal's session/token endpoint.
+    return fetch('/session/token', {
+      method: 'GET',
+      credentials: 'same-origin',
+    })
+    .then(function(response) {
+      if (!response.ok) {
+        throw new Error('Failed to get CSRF token');
+      }
+      return response.text();
+    })
+    .then(function(token) {
+      return token.trim();
+    });
+  }
+
+})(Drupal);