diff --git a/INSTALL b/INSTALL
index 58ec0b60a..48b125049 100644
--- a/INSTALL
+++ b/INSTALL
@@ -78,8 +78,8 @@ Or you can use one of the "projects" in the "projects" directory.
 
 Before installing libpng, you must first install zlib, if it
 is not already on your system.  zlib can usually be found
-wherever you got libpng; otherwise go to http://zlib.net.  You can place
-zlib in the same directory as libpng or in another directory.
+wherever you got libpng; otherwise go to https://zlib.net/.  You can
+place zlib in the same directory as libpng or in another directory.
 
 If your system already has a preinstalled zlib you will still need
 to have access to the zlib.h and zconf.h include files that
diff --git a/contrib/gregbook/README b/contrib/gregbook/README
index 4d7be4260..90e28f7ce 100644
--- a/contrib/gregbook/README
+++ b/contrib/gregbook/README
@@ -63,7 +63,7 @@ BUILD INSTRUCTIONS
 
  - Prerequisites (in order of compilation):
 
-      - zlib            http://zlib.net/
+      - zlib            https://zlib.net/
       - libpng          http://www.libpng.org/pub/png/libpng.html
       - pngbook         http://www.libpng.org/pub/png/book/sources.html
 
diff --git a/libpng-manual.txt b/libpng-manual.txt
index 1f1556805..81b17fbfd 100644
--- a/libpng-manual.txt
+++ b/libpng-manual.txt
@@ -101,7 +101,7 @@ majority of the needs of its users.
 
 Libpng uses zlib for its compression and decompression of PNG files.
 Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://zlib.net/>.
+be found at the zlib home page, <https://zlib.net/>.
 The zlib compression utility is a general purpose utility that is
 useful for more than PNG files, and can be used without libpng.
 See the documentation delivered with zlib for more details.
diff --git a/libpng.3 b/libpng.3
index 4134dabb7..2237250c6 100644
--- a/libpng.3
+++ b/libpng.3
@@ -611,7 +611,7 @@ majority of the needs of its users.
 
 Libpng uses zlib for its compression and decompression of PNG files.
 Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://zlib.net/>.
+be found at the zlib home page, <https://zlib.net/>.
 The zlib compression utility is a general purpose utility that is
 useful for more than PNG files, and can be used without libpng.
 See the documentation delivered with zlib for more details.
@@ -6016,7 +6016,7 @@ http://www.libpng.org/pub/png
 .I libpng
 or at
 .br
-http://zlib.net/
+https://zlib.net/
 
 .LP
 .IR PNG specification: RFC 2083