James Maslow -- From 'Big Time Rush' to Big Time Crush

Former Nickelodeon boy band star James Maslow no longer looks like Donny Osmond's stunt double ... these days he could nearly pass for Captain America.

Maslow's transformation in the last year is unbelievable ... going from some 70s mop-haired baby-faced cheeseball to a Chris Evans look-alike in a matter of months.

A good move ... especially if he wants to keep getting jobs in Hollywood ... and not at some Atlantic City variety show.