Hopfensee is a lake in Swabia, Bavaria, Germany.

Its maximum depth is 10 m. It was formed by the Lechtal glacier and is one of the remains of a larger Fuessener See.

The Hopfensee-Ach flows out of the lake and into the Forggensee.

Besides swimming, which is possible as early as the end of May, boating, sailing, and wind surfing are common activities.

The north shore has extensive views of the Allgäu Alps.