diff --git a/humanfriendly/__init__.py b/humanfriendly/__init__.py
index 4c0a333..212e503 100644
--- a/humanfriendly/__init__.py
+++ b/humanfriendly/__init__.py
@@ -429,13 +429,14 @@ def format_timespan(num_seconds, detailed=False, max_units=3):
     '1 year, 2 days and 3 hours'
     """
     num_seconds = coerce_seconds(num_seconds)
-    if num_seconds < 60 and not detailed:
+    if abs(num_seconds) < 60 and not detailed:
         # Fast path.
         return pluralize(round_number(num_seconds), 'second')
     else:
         # Slow path.
+        is_negative = num_seconds < 0
         result = []
-        num_seconds = decimal.Decimal(str(num_seconds))
+        num_seconds = decimal.Decimal(str(abs(num_seconds)))
         relevant_units = list(reversed(time_units[0 if detailed else 3:]))
         for unit in relevant_units:
             # Extract the unit count from the remaining time.
@@ -454,13 +455,16 @@ def format_timespan(num_seconds, detailed=False, max_units=3):
                 result.append(pluralize(count, unit['singular'], unit['plural']))
         if len(result) == 1:
             # A single count/unit combination.
-            return result[0]
+            formatted = result[0]
         else:
             if not detailed:
                 # Remove `insignificant' data from the formatted timespan.
                 result = result[:max_units]
             # Format the timespan in a readable way.
-            return concatenate(result)
+            formatted = concatenate(result)
+        # Apply a single leading minus sign for negative timespans instead of
+        # letting each individual unit carry its own sign.
+        return ('-' + formatted) if is_negative else formatted
 
 
 def parse_timespan(timespan):
diff --git a/humanfriendly/tests.py b/humanfriendly/tests.py
index 72dad99..eadcb70 100644
--- a/humanfriendly/tests.py
+++ b/humanfriendly/tests.py
@@ -448,6 +448,12 @@ def test_format_timespan(self):
         assert '5 minutes and 0.3 seconds' == format_timespan(300.300)
         assert '1 second and 15 milliseconds' == format_timespan(1.015, detailed=True)
         assert '10 seconds and 15 milliseconds' == format_timespan(10.015, detailed=True)
+        # Negative timespans should get a single leading minus sign instead
+        # of a minus sign on every individual unit.
+        assert '-1 hour, 1 minute and 40 seconds' == format_timespan(-3700, detailed=True)
+        assert '-1 minute and 30 seconds' == format_timespan(-90, detailed=True)
+        assert '-30 seconds' == format_timespan(-30, detailed=False)
+        assert '-1 hour, 1 minute and 40 seconds' == format_timespan(-3700, detailed=False)
         assert '1 microsecond and 50 nanoseconds' == format_timespan(0.00000105, detailed=True)
         # Test the datetime.timedelta support:
         # https://github.com/xolox/python-humanfriendly/issues/27