Police are seeking the public's help in identifying an assailant who grabbed a woman's thigh on the D train, and then threatened a couple with a box cutter.

The suspect approached his first victim on a southbound D train about 12:30 p.m. July 30, and touched her leg.

When he got off the station, he flashed the blade at a woman and her boyfriend on the southbound platform.

He is 6 feet tall, and was wearing a white polo shirt and yellow shorts that day, police said.

Cops are also looking for two robbers who stole a woman's smartphone on the No. 7 train -- and pushed her down the stairs when she chased after them.

They grabbed the phone about 1:50 p.m. on Aug. 22 at the Junction Boulevard station. She followed them when they fled from the train, and they threw her down the steps.

The victim was treated at Elmhurst Hospital, authorities added.

Police are asking anyone who recognizes the assailants in the crimes to call 1-800-577-TIPS, or visit www.nypdcrimestoppers.com.