diff --git a/bignum.c b/bignum.c
index 4ab117b557..142059b8fd 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1573,7 +1573,7 @@ rb_big_mul_normal(VALUE x, VALUE y)
 
 /* efficient squaring (2 times faster than normal multiplication)
  * ref: Handbook of Applied Cryptography, Algorithm 14.16
- *      http://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf
+ *      https://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf
  */
 static void
 bary_sq_fast(BDIGIT *zds, size_t zn, const BDIGIT *xds, size_t xn)
diff --git a/configure.ac b/configure.ac
index c856aa4484..cbc53231d8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2448,7 +2448,7 @@ AC_CHECK_TYPES([sig_t],[],[],[@%:@include <signal.h>])
 AS_IF([test "$ac_cv_func_getpgid" = no], [
   # AC_FUNC_GETPGRP fails when cross-compiling with old autoconf.
   # autoconf is changed between 2.52d and 2.52f?
-  # http://lists.gnu.org/archive/html/bug-gnu-utils/2001-09/msg00181.html
+  # https://lists.gnu.org/archive/html/bug-gnu-utils/2001-09/msg00181.html
   # "autoconf cleanup for AC_FUNC_GETPGRP and GETPGRP_VOID"
 AC_FUNC_GETPGRP
 ])
diff --git a/gc.c b/gc.c
index 4c88be0df1..2a2d1a169a 100644
--- a/gc.c
+++ b/gc.c
@@ -1275,7 +1275,7 @@ static const char *obj_type_name(VALUE obj);
 /* the following code is only for internal tuning. */
 
 /* Source code to use RDTSC is quoted and modified from
- * http://www.mcs.anl.gov/~kazutomo/rdtsc.html
+ * https://www.mcs.anl.gov/~kazutomo/rdtsc.html
  * written by Kazutomo Yoshii <kazutomo@mcs.anl.gov>
  */
 
@@ -11806,7 +11806,7 @@ objspace_xrealloc(rb_objspace_t *objspace, void *ptr, size_t new_size, size_t ol
              *   a non-NULL pointer when its argument is 0.  That return value
              *   is safe (and is expected) to be passed to free().
              *
-             *   http://man7.org/linux/man-pages/man3/malloc.3.html
+             *   https://man7.org/linux/man-pages/man3/malloc.3.html
              *
              * - As I read the implementation jemalloc's malloc() returns fully
              *   normal 16 bytes memory region when its argument is 0.
diff --git a/sparc.c b/sparc.c
index 7f3a70fd0e..fe9dd684f4 100644
--- a/sparc.c
+++ b/sparc.c
@@ -7,7 +7,7 @@
  window of the process executing the instruction will not be flushed
  correctly.
 
- See http://bugs.ruby-lang.org/issues/5244 for discussion.
+ See https://bugs.ruby-lang.org/issues/5244 for discussion.
 *********************************************************************/
 void
 rb_sparc_flush_register_windows(void)
@@ -15,7 +15,7 @@ rb_sparc_flush_register_windows(void)
 /*
  * gcc doesn't provide "asm" keyword if -ansi and the various -std options
  * are given.
- * http://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html
+ * https://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html
  */
 #ifndef __GNUC__
 #define __asm__ asm
diff --git a/time.c b/time.c
index e7faef6df6..8b4d26b2dc 100644
--- a/time.c
+++ b/time.c
@@ -1315,7 +1315,7 @@ static struct tm *localtime_with_gmtoff_zone(const time_t *t, struct tm *result,
  * compat_common_month_table is generated by the following program.
  * This table finds the last month which starts at the same day of a week.
  * The year 2037 is not used because:
- * http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=522949
+ * https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=522949
  *
  *  #!/usr/bin/ruby
  *
diff --git a/vm_dump.c b/vm_dump.c
index d7223e9ef9..271879ce18 100644
--- a/vm_dump.c
+++ b/vm_dump.c
@@ -507,8 +507,8 @@ darwin_sigtramp:
 	char vec[1];
 	int r;
 	/* get previous frame information from %rbx at _sigtramp and set values to cursor
-	 * http://www.opensource.apple.com/source/Libc/Libc-825.25/i386/sys/_sigtramp.s
-	 * http://www.opensource.apple.com/source/libunwind/libunwind-35.1/src/unw_getcontext.s
+	 * https://www.opensource.apple.com/source/Libc/Libc-825.25/i386/sys/_sigtramp.s
+	 * https://www.opensource.apple.com/source/libunwind/libunwind-35.1/src/unw_getcontext.s
 	 */
 	unw_get_reg(&cursor, UNW_X86_64_RBX, &ip);
 	uctx = (ucontext_t *)ip;