The star wideout recently joined his head coach, Sean McVay, and two other Rams players to film a video for the LASD in an effort to help the org recruit more talent.

Kupp, McVay, Steve Avila and Quentin Lake all appeared in the clip ... with each going on-camera to encourage citizens to consider joining the Dept.

At one point during the vid, Avila -- a second-year offensive lineman -- called the LASD "the best team in Los Angeles."

Law enforcement tells TMZ Sports -- reps for the LASD actually reached out to Kupp about the gig ... after recent recruiting efforts had been lacking.

Kupp, of course, had already built a good relationship with the deputies -- bringing them free BBQ during the holidays -- and was happy to oblige.

We're told all of the NFLers now have open invites to come hang out at the Department any time they'd like -- and participate in any training simulations that they want to experience.