Oakland Raiders star Aldon Smith is being investigated for what we're told is a domestic incident ... TMZ Sports has learned.

Sources tell us cops in San Francisco were called early Saturday morning and responded to a home in the city. Smith owns a house in San Francisco.

Police interviewed Smith and the alleged victim and took a report. The case is under "active investigation."

Smith has not been arrested.

An arrest would be catastrophic for Smith's career -- he's currently suspended by the NFL for multiple violations of the league's substance abuse policy.

Smith was expected to be reinstated in March, on the assumption he stayed out of trouble. An arrest could undo his prospects.