Cowboys coach Jason Garrett announced the signing Wednesday morning.
With a definite need for pass-rush help, the Cowboys are bringing in Sam, who had three sacks in the preseason for the St. Louis Rams, for an extended look to see if last year's co-SEC Defensive Player of the Year at Missouri can potentially help a defensive line that needs help.
Sam will be assigned No. 46 with the Cowboys, sources told ESPN. This will complicate any potential jersey sales, since Sam will not wear that number if he is activated to the Cowboys' 53-man roster at any point.
Any team can sign Sam off the Cowboys' practice squad, but would have to put him on the 53-man roster for a minimum of four weeks.
Sam was among final group of players released last weekend by the Rams and went unclaimed off waivers. The Cowboys brought the defensive end in for a physical, and now will see what he can provide.
Sam was not on the Cowboys' draft board in May. Dallas had five seventh-round picks and did not select him.
ESPN.com Cowboys reporter Todd Archer, ESPN.com's Darren Rovell and The Associated Press contributed to this report.