Steelers LB Anthony Chickillo is off the hook after he allegedly roughed up his track star girlfriend ... prosecutors tell TMZ Sports she refused to testify against the NFL player, so the charges were dropped.

As we previously reported, Chickillo was arrested on Oct. 20 after his GF -- Canadian track star Alysha Newman -- said an argument at a fancy Pennsylvania resort's casino later turned violent in their hotel room.

Newman claimed Chickillo grabbed her by both arms, forced her against the wall and door and eventually threw her to the ground while intending to cause her injury.

In court records obtained by TMZ Sports, cops say they noticed Newman "had visible redness to both her right and left bicep areas, with bruising beginning to form."

Chickillo was eventually taken into custody and hit with three charges -- simple assault, criminal mischief and harassment.

For her part in the altercation, cops also hit Newman with a summary citation for harassment.

But, prosecutors tell us after interviewing both Newman and Chickillo at length, both parties said they would refuse to testify against each other and did NOT want to pursue charges.

So, at Chickillo's hearing in court Wednesday ... prosecutors announced they dropped the charges.

Anthony returned to work afterward ... showing up to the Steelers' facility, but it's unclear when he'll be able to play again as he's still on the Commissioner's Exempt List.

Of course, even with the charges dropped, the NFL can still suspend the linebacker if it finds in its own independent investigation that he did violate its personal conduct policy.

Chickillo has been a regular on the Steelers' roster over the past five years ... playing in 57 games and recording 7.5 sacks.