He committed the ultimate crime — of fashion.

A Brooklyn cellphone crook’s poor wardrobe choices led to his arrest when his saggy jeans tripped him up during his getaway attempt and allowed cops to chase him down, police said.

Joel Donaldson, 21, allegedly punched his victim in the face before snatching her phone at around 2:30 p.m. at Court and Remsen streets, just steps from Borough Hall. He then tried to get away on foot, but didn’t get far.

His droopy blue jeans — which left his boxer shorts exposed — kept slipping down as he ran.

A cop who was directing traffic nearby spotted the bungling bad guy’s sorry sartorial situation and hurried after him.

Donaldson made it only about a block before his pants were completely around his ankles, allowing the officer to tackle him near Joralemon Street.

“He was zigzagging all over the place, but he couldn’t run because his pants was falling down,” witness Arlene Williams said.

“This cop saw it, and he went right after him.’’

Donaldson was arrested two blocks from Brooklyn Criminal Court and charged with robbery, cops said.