diff --git i/array.c w/array.c
index ce36e19..56adacf 100644
--- i/array.c
+++ w/array.c
@@ -1730,8 +1730,8 @@ rb_ary_join(VALUE ary, VALUE sep)
     VALUE val, tmp, result;
 
     if (RARRAY_LEN(ary) == 0) return rb_usascii_str_new(0, 0);
-    if (OBJ_TAINTED(ary) || OBJ_TAINTED(sep)) taint = TRUE;
-    if (OBJ_UNTRUSTED(ary) || OBJ_UNTRUSTED(sep)) untrust = TRUE;
+    if (OBJ_TAINTED(ary)) taint = TRUE;
+    if (OBJ_UNTRUSTED(ary)) untrust = TRUE;
 
     if (!NIL_P(sep)) {
 	StringValue(sep);
diff --git i/test/ruby/test_array.rb w/test/ruby/test_array.rb
index 28d3e41..801411e 100644
--- i/test/ruby/test_array.rb
+++ w/test/ruby/test_array.rb
@@ -920,6 +920,19 @@ class TestArray < Test::Unit::TestCase
     assert_equal(true, s.tainted?)
     assert_equal(true, s.untrusted?)
 
+    bug5902 = '[ruby-core:42161]'
+    sep = ":".taint.untrust
+
+    s = @cls[].join(sep)
+    assert_equal(false, s.tainted?, bug5902)
+    assert_equal(false, s.untrusted?, bug5902)
+    s = @cls[1].join(sep)
+    assert_equal(false, s.tainted?, bug5902)
+    assert_equal(false, s.untrusted?, bug5902)
+    s = @cls[1, 2].join(sep)
+    assert_equal(true, s.tainted?, bug5902)
+    assert_equal(true, s.untrusted?, bug5902)
+
     e = ''.force_encoding('EUC-JP')
     u = ''.force_encoding('UTF-8')
     assert_equal(Encoding::US_ASCII, [[]].join.encoding)