diff --git a/INSTALL b/INSTALL
index 58ec0b6..48b1250 100644
--- a/INSTALL
+++ b/INSTALL
@@ -78,8 +78,8 @@
 
 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 4d7be42..90e28f7 100644
--- a/contrib/gregbook/README
+++ b/contrib/gregbook/README
@@ -63,7 +63,7 @@
 
  - 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 1f15568..81b17fb 100644
--- a/libpng-manual.txt
+++ b/libpng-manual.txt
@@ -101,7 +101,7 @@
 
 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 4134dab..2237250 100644
--- a/libpng.3
+++ b/libpng.3
@@ -611,7 +611,7 @@
 
 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 @@
 .I libpng
 or at
 .br
-http://zlib.net/
+https://zlib.net/
 
 .LP
 .IR PNG specification: RFC 2083