A grease gun with an adjustable operating angle includes: a grease barrel having a first annular assembling portion and a first annular pivot portion; a gun body having a second annular assembling portion facing the first annular assembling portion and a second annular pivot portion to be pivotally disposed on the first annular pivot portion; a positioning ring fixed to the second annular assembling portion and having annularly arranged positioning cavities and facing the first annular assembling portion; and a positioning unit having a positioning member movable between a locked position and a release position, and the positioning member having a positioning portion to be inserted into or disengaged from the positioning cavities. The grease barrel and the gun body can be rotated and fixed relative to each other after rotation to adjust the angle between the two, which consequently reduces the space limitation on the grease feeding operation.