Amanda Bynes was arrested for DUI in West Hollywood early this morning ... TMZ has learned.

We're told the 26-year-old actress was busted at 3 AM ... she's still behind bars.

According to her booking sheet, Bynes is 5'7" and 125 lbs.

Her bail has been set at $5,000.

Bynes has been frequenting the Hollywood party scene recently, frequenting several clubs over the last few weeks.

Just last month -- on St. Patrick's Day -- TMZ cameras saw Bynes leaving Greystone Manor, and thought she appeared to be extremely intoxicated. One of our cameramen even warned the valet not to let Amanda drive, but she got into the driver's seat anyway ... and took off.