diff --git a/tmux.1 b/tmux.1
index b0fc5600..e785cf8b 100644
--- a/tmux.1
+++ b/tmux.1
@@ -565,6 +565,18 @@ Environment variables may be set by using the syntax
 for example
 .Ql HOME=/home/user .
 Variables set during parsing are added to the global environment.
+A hidden variable may be set with
+.Ql %hidden ,
+for example:
+.Bd -literal -offset indent
+%hidden MYVAR=42
+.Ed
+.Pp
+Hidden variables are not passed to the environment of processes created
+by tmux.
+See the
+.Sx GLOBAL AND SESSION ENVIRONMENT
+section.
 .Pp
 Commands may be parsed conditionally by surrounding them with
 .Ql %if ,
@@ -4711,10 +4723,16 @@ from inside, and the
 variable with the correct terminal setting of
 .Ql screen .
 .Pp
+Variables in both session and global environments may be marked as hidden.
+Hidden variables are not passed into the environment of new processes and
+instead can only be used by tmux itself (for example in formats, see the
+.Sx FORMATS
+section).
+.Pp
 Commands to alter and view the environment are:
 .Bl -tag -width Ds
 .It Xo Ic set-environment
-.Op Fl gru
+.Op Fl hgru
 .Op Fl t Ar target-session
 .Ar name Op Ar value
 .Xc
@@ -4731,8 +4749,10 @@ flag unsets a variable.
 .Fl r
 indicates the variable is to be removed from the environment before starting a
 new process.
+.Fl h
+marks the variable as hidden.
 .It Xo Ic show-environment
-.Op Fl gs
+.Op Fl hgs
 .Op Fl t Ar target-session
 .Op Ar variable
 .Xc
@@ -4749,6 +4769,8 @@ Variables removed from the environment are prefixed with
 If
 .Fl s
 is used, the output is formatted as a set of Bourne shell commands.
+.Fl h
+shows hidden variables (omitted by default).
 .El
 .Sh STATUS LINE
 .Nm
diff --git a/tmux.h b/tmux.h
index 20559144..53bbc222 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1048,6 +1048,9 @@ struct environ_entry {
 	char		*name;
 	char		*value;
 
+	int		 flags;
+#define ENVIRON_HIDDEN 0x1
+
 	RB_ENTRY(environ_entry) entry;
 };
 
@@ -1957,10 +1960,10 @@ struct environ_entry *environ_first(struct environ *);
 struct environ_entry *environ_next(struct environ_entry *);
 void	environ_copy(struct environ *, struct environ *);
 struct environ_entry *environ_find(struct environ *, const char *);
-void printflike(3, 4) environ_set(struct environ *, const char *, const char *,
-	    ...);
+void printflike(4, 5) environ_set(struct environ *, const char *, int,
+	    const char *, ...);
 void	environ_clear(struct environ *, const char *);
-void	environ_put(struct environ *, const char *);
+void	environ_put(struct environ *, const char *, int);
 void	environ_unset(struct environ *, const char *);
 void	environ_update(struct options *, struct environ *, struct environ *);
 void	environ_push(struct environ *);