diff --git a/.github/workflows/posix-deps-apt.sh b/.github/workflows/posix-deps-apt.sh
index 7773222af5d26f..44e6a9ce2d0cd1 100755
--- a/.github/workflows/posix-deps-apt.sh
+++ b/.github/workflows/posix-deps-apt.sh
@@ -25,3 +25,10 @@ apt-get -yq install \
     uuid-dev \
     xvfb \
     zlib1g-dev
+
+# Workaround missing libmpdec-dev on ubuntu 24.04:
+# https://launchpad.net/~ondrej/+archive/ubuntu/php
+# https://deb.sury.org/
+sudo add-apt-repository ppa:ondrej/php
+apt-get update
+apt-get -yq install libmpdec-dev
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 9f327cf904da1b..9084f6e5aaa8eb 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -293,6 +293,11 @@ that may require changes to your code.
 Build changes
 =============
 
+* Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
+  Now this should be explicitly enabled with :option:`--with-system-libmpdec`
+  set to ``no`` or with :option:`!--without-system-libmpdec`.
+  (Contributed by Sergey B Kirpichev in :gh:`115119`.)
+
 
 C API changes
 =============
diff --git a/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst b/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst
new file mode 100644
index 00000000000000..8c2d15a3228fcf
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst
@@ -0,0 +1,4 @@
+Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
+Now this should be explicitly enabled via :option:`--with-system-libmpdec`
+set to ``no`` or :option:`!--without-system-libmpdec`.  Patch by Sergey
+B Kirpichev.
diff --git a/configure b/configure
index 2b1482d9264d24..c2467c9da449b5 100755
--- a/configure
+++ b/configure
@@ -15757,10 +15757,18 @@ fi
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for --with-system-libmpdec" >&5
 printf %s "checking for --with-system-libmpdec... " >&6; }
 
+
 # Check whether --with-system_libmpdec was given.
 if test ${with_system_libmpdec+y}
 then :
-  withval=$with_system_libmpdec;
+  withval=$with_system_libmpdec; if test "x$with_system_libmpdec" = xno
+then :
+  LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
+          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
+          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
+          have_mpdec=yes
+          with_system_libmpdec=no
+fi
 else case e in #(
   e) with_system_libmpdec="yes" ;;
 esac
@@ -15769,8 +15777,6 @@ fi
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $with_system_libmpdec" >&5
 printf "%s\n" "$with_system_libmpdec" >&6; }
 
-
-
 if test "x$with_system_libmpdec" = xyes
 then :
 
@@ -15848,13 +15854,6 @@ else
 printf "%s\n" "yes" >&6; }
 
 fi
-else case e in #(
-  e) LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
-          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
-          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
-          have_mpdec=yes
-          with_system_libmpdec=no ;;
-esac
 fi
 
 if test "x$with_system_libmpdec" = xyes
@@ -15901,21 +15900,6 @@ LDFLAGS=$save_LDFLAGS
 LIBS=$save_LIBS
 
 
-else case e in #(
-  e) { printf "%s\n" "$as_me:${as_lineno-$LINENO}: WARNING: the bundled copy of libmpdecimal is scheduled for removal in Python 3.15; consider using a system installed mpdecimal library." >&5
-printf "%s\n" "$as_me: WARNING: the bundled copy of libmpdecimal is scheduled for removal in Python 3.15; consider using a system installed mpdecimal library." >&2;} ;;
-esac
-fi
-
-if test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"
-then :
-  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: WARNING: no system libmpdecimal found; falling back to bundled libmpdecimal (deprecated and scheduled for removal in Python 3.15)" >&5
-printf "%s\n" "$as_me: WARNING: no system libmpdecimal found; falling back to bundled libmpdecimal (deprecated and scheduled for removal in Python 3.15)" >&2;}
-       LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
-          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
-          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
-          have_mpdec=yes
-          with_system_libmpdec=no
 fi
 
 # Disable forced inlining in debug builds, see GH-94847
@@ -33204,6 +33188,18 @@ fi
 printf "%s\n" "$py_cv_module__decimal" >&6; }
 
 
+if test "x$with_system_libmpdec" = xno
+then :
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: WARNING: the bundled copy of libmpdecimal is scheduled for removal in Python 3.16; consider using a system installed mpdecimal library." >&5
+printf "%s\n" "$as_me: WARNING: the bundled copy of libmpdecimal is scheduled for removal in Python 3.16; consider using a system installed mpdecimal library." >&2;}
+fi
+if test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"
+then :
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: WARNING: no system libmpdecimal found; falling back to pure-Python version for the decimal module" >&5
+printf "%s\n" "$as_me: WARNING: no system libmpdecimal found; falling back to pure-Python version for the decimal module" >&2;}
+fi
+
+
   { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for stdlib extension module _dbm" >&5
 printf %s "checking for stdlib extension module _dbm... " >&6; }
         if test "$py_cv_module__dbm" != "n/a"
diff --git a/configure.ac b/configure.ac
index 7dbb332f1d30a7..bc671cd4c7e3cf 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4173,31 +4173,30 @@ fi
 
 # Check for use of the system libmpdec library
 AC_MSG_CHECKING([for --with-system-libmpdec])
+AC_DEFUN([USE_BUNDLED_LIBMPDEC],
+         [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
+          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
+          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
+          have_mpdec=yes
+          with_system_libmpdec=no])
 AC_ARG_WITH(
   [system_libmpdec],
   [AS_HELP_STRING(
     [--with-system-libmpdec],
     [build _decimal module using an installed mpdecimal library, see Doc/library/decimal.rst (default is yes)]
   )],
-  [],
+  [AS_IF([test "x$with_system_libmpdec" = xno],
+         [USE_BUNDLED_LIBMPDEC()])],
   [with_system_libmpdec="yes"])
 AC_MSG_RESULT([$with_system_libmpdec])
 
-AC_DEFUN([USE_BUNDLED_LIBMPDEC],
-         [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
-          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
-          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
-          have_mpdec=yes
-          with_system_libmpdec=no])
-
 AS_VAR_IF(
   [with_system_libmpdec], [yes],
   [PKG_CHECK_MODULES(
     [LIBMPDEC], [libmpdec >= 2.5.0], [],
     [LIBMPDEC_CFLAGS=${LIBMPDEC_CFLAGS-""}
      LIBMPDEC_LIBS=${LIBMPDEC_LIBS-"-lmpdec -lm"}
-     LIBMPDEC_INTERNAL=])],
-  [USE_BUNDLED_LIBMPDEC()])
+     LIBMPDEC_INTERNAL=])])
 
 AS_VAR_IF([with_system_libmpdec], [yes],
   [WITH_SAVE_ENV([
@@ -4213,16 +4212,7 @@ AS_VAR_IF([with_system_libmpdec], [yes],
       ], [const char *x = mpd_version();])],
       [have_mpdec=yes],
       [have_mpdec=no])
-  ])],
-  [AC_MSG_WARN([m4_normalize([
-     the bundled copy of libmpdecimal is scheduled for removal in Python 3.15;
-     consider using a system installed mpdecimal library.])])])
-
-AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
-      [AC_MSG_WARN([m4_normalize([
-         no system libmpdecimal found; falling back to bundled libmpdecimal
-         (deprecated and scheduled for removal in Python 3.15)])])
-       USE_BUNDLED_LIBMPDEC()])
+  ])])
 
 # Disable forced inlining in debug builds, see GH-94847
 AS_VAR_IF(
@@ -8140,6 +8130,16 @@ PY_STDLIB_MOD([_curses_panel],
 PY_STDLIB_MOD([_decimal],
   [], [test "$have_mpdec" = "yes"],
   [$LIBMPDEC_CFLAGS], [$LIBMPDEC_LIBS])
+
+AS_VAR_IF([with_system_libmpdec], [no],
+          [AC_MSG_WARN([m4_normalize([
+     the bundled copy of libmpdecimal is scheduled for removal in Python 3.16;
+     consider using a system installed mpdecimal library.])])])
+AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
+      [AC_MSG_WARN([m4_normalize([
+         no system libmpdecimal found; falling back to pure-Python version
+         for the decimal module])])])
+
 PY_STDLIB_MOD([_dbm],
   [test -n "$with_dbmliborder"], [test "$have_dbm" != "no"],
   [$DBM_CFLAGS], [$DBM_LIBS])