Abstract: A liquid filter has a filter housing with a housing cup. A filter element of the liquid filter has a filter medium annularly surrounding a longitudinal axis of the filter element. The filter element has an extension part with lock groove. A lock device arranged at the housing cup has a lock element movable radially to the longitudinal axis. The lock element engages the lock groove in a locked state of the lock device and is disengaged in a release state. A filter element for the liquid filter has a filter medium annularly surrounding a longitudinal axis and an extension part with lock groove with rounded cross section. The lock groove is engaged by a lock element, movable radially relative to the longitudinal axis, of a lock device of a housing cup of the liquid filter in a locked state of the lock device and disengaged in a release state.