diff --git a/.github/ISSUE_TEMPLATE.md b/.github/ISSUE_TEMPLATE.md
index d081c072702..38fabbfdb71 100644
--- a/.github/ISSUE_TEMPLATE.md
+++ b/.github/ISSUE_TEMPLATE.md
@@ -25,4 +25,4 @@ If this is a **feature request**, show what you expect to happen if the feature
 
 
 #### Current Version:
-v4.2.0
+v4.2.1
diff --git a/RELEASE-NOTES.txt b/RELEASE-NOTES.txt
index 24668d6ebd3..bfe51740bca 100644
--- a/RELEASE-NOTES.txt
+++ b/RELEASE-NOTES.txt
@@ -1,5 +1,12 @@
 Stan Math Library Release Notes
 
+======================================================================
+v4.2.1 (21 October 2021)
+======================================================================
+
+- Fixed issues with Adjoint ODE memory management
+- Fixed included OpenCL headers
+
 ======================================================================
 v4.2.0 (5 October 2021)
 ======================================================================
diff --git a/doxygen/doxygen.cfg b/doxygen/doxygen.cfg
index b732ff5f03f..60f7df23510 100644
--- a/doxygen/doxygen.cfg
+++ b/doxygen/doxygen.cfg
@@ -38,7 +38,7 @@ PROJECT_NAME           = "Stan Math Library"
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 4.2.0
+PROJECT_NUMBER         = 4.2.1
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a
diff --git a/stan/math/version.hpp b/stan/math/version.hpp
index 0af5a5b6722..e81edbd5f04 100644
--- a/stan/math/version.hpp
+++ b/stan/math/version.hpp
@@ -13,7 +13,7 @@
 
 #define STAN_MATH_MAJOR 4
 #define STAN_MATH_MINOR 2
-#define STAN_MATH_PATCH 0
+#define STAN_MATH_PATCH 1
 
 namespace stan {
 namespace math {