diff --git a/lisp/international/characters.el b/lisp/international/characters.el
index 0b6920cf180..5f610ddf670 100644
--- a/lisp/international/characters.el
+++ b/lisp/international/characters.el
@@ -759,7 +759,14 @@ with L, LRE, or LRO Unicode bidi character type.")
         (funcall map-unicode-property 'uppercase
                  (lambda (lc uc) (aset up lc uc) (aset up uc uc)))
         (funcall map-unicode-property 'lowercase
-                 (lambda (uc lc) (aset down uc lc) (aset down lc lc))))))
+                 (lambda (uc lc) (aset down uc lc) (aset down lc lc)))
+
+        ;; Override the Unicode uppercase property for ß, since we are
+        ;; using our case tables for determining the case of a
+        ;; character (see uppercasep and lowercasep in buffer.h).
+        ;; The special-uppercase property of ß ensures that it is
+        ;; still upcased to SS per the usual convention.
+        (aset up ?ß ?ẞ))))
 
   ;; Clear out the extra slots so that they will be recomputed from the main
   ;; (downcase) table and upcase table.  Since we’re side-stepping the usual
diff --git a/test/src/regex-emacs-tests.el b/test/src/regex-emacs-tests.el
index 576630aa5af..34d4067db47 100644
--- a/test/src/regex-emacs-tests.el
+++ b/test/src/regex-emacs-tests.el
@@ -834,6 +834,13 @@ This evaluates the TESTS test cases from glibc."
 
 (ert-deftest regexp-eszett ()
   "Test matching of ß and ẞ."
+  ;; Sanity checks.
+  (should (equal (upcase "ß") "SS"))
+  (should (equal (downcase "ß") "ß"))
+  (should (equal (capitalize "ß") "Ss"))  ; undeutsch...
+  (should (equal (upcase "ẞ") "ẞ"))
+  (should (equal (downcase "ẞ") "ß"))
+  (should (equal (capitalize "ẞ") "ẞ"))
   ;; ß is a lower-case letter (Ll); ẞ is an upper-case letter (Lu).
   (let ((case-fold-search nil))
     (should (equal (string-match "ß" "ß") 0))
@@ -842,8 +849,8 @@ This evaluates the TESTS test cases from glibc."
     (should (equal (string-match "ẞ" "ẞ") 0))
     (should (equal (string-match "[[:alpha:]]" "ß") 0))
     ;; bug#11309
-    ;;(should (equal (string-match "[[:lower:]]" "ß") 0))
-    ;;(should (equal (string-match "[[:upper:]]" "ß") nil))
+    (should (equal (string-match "[[:lower:]]" "ß") 0))
+    (should (equal (string-match "[[:upper:]]" "ß") nil))
     (should (equal (string-match "[[:alpha:]]" "ẞ") 0))
     (should (equal (string-match "[[:lower:]]" "ẞ") nil))
     (should (equal (string-match "[[:upper:]]" "ẞ") 0)))
@@ -854,8 +861,8 @@ This evaluates the TESTS test cases from glibc."
     (should (equal (string-match "ẞ" "ẞ") 0))
     (should (equal (string-match "[[:alpha:]]" "ß") 0))
     ;; bug#11309
-    ;;(should (equal (string-match "[[:lower:]]" "ß") 0))
-    ;;(should (equal (string-match "[[:upper:]]" "ß") 0))
+    (should (equal (string-match "[[:lower:]]" "ß") 0))
+    (should (equal (string-match "[[:upper:]]" "ß") 0))
     (should (equal (string-match "[[:alpha:]]" "ẞ") 0))
     (should (equal (string-match "[[:lower:]]" "ẞ") 0))
     (should (equal (string-match "[[:upper:]]" "ẞ") 0))))