2:30 PM PT -- TMZ Sports has obtained Ross' mug shot ... and the dude looks sad.
Dallas Cowboys defensive lineman Daniel Ross was arrested in Frisco, TX on Wednesday ... after cops allegedly found weed and a gun in his car.
TMZ Sports has confirmed ... the 26-year-old (who signed a 1-year deal with Dallas in the offseason) was pulled over by Frisco PD early Wednesday morning during a routine traffic stop ... and at some point, cops say they found the weed and the firearm.
We're working on getting more info about the incident.