EXCLUSIVE

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.

Smith has had several run-ins with police since he was drafted by the 49ers back in 2011.

Smith showed extreme potential when he was on the field -- he was the NFC Defensive Player of the Year back in 2012.