A boysenberry /ˈbɔɪzənbɛri/ is a cross between a European Raspberry (Rubus idaeus), a Common Blackberry (Rubus fruticosus), and a Loganberry (Rubus × loganobaccus).

It is a large (8.0 g/0.28 oz) aggregate fruit, with large seeds and a deep maroon color, which changes to the typical boysenberry color when the fruit is cooked and made into jam and pies.

In the late 1920s, George M. Darrow of the USDA began tracking down reports of a large, reddish-purple berry that had been grown on the northern California farm of a man named Rudolph Boysen. Darrow enlisted the help of Walter Knott, a Southern California farmer who was known as a berry expert. Knott had never heard of the new berry, but he agreed to help Darrow in his search.

Darrow and Knott learned that Boysen had abandoned his growing experiments several years earlier and sold his farm. Undaunted by this news, Darrow and Knott headed out to Boysen's old farm, on which they found several frail vines surviving in a field choked with weeds. They transplanted the vines to Knott's farm in Buena Park, California, where he nurtured them back to fruit-bearing health. Walter Knott was the first to commercially cultivate the berry in southern California. He began selling the berries at his farm stand in 1932 and soon noticed that people kept returning to buy the large, tasty berries. When asked what they were called, Knott said, "Boysenberries," after their originator. His family's small restaurant and pie business eventually grew into Knott's Berry Farm. As the berry's popularity grew, Mrs. Knott began making preserves, which ultimately made Knott's Berry Farm famous.