Beljan, the 2002 U.S. Junior Amateur champion, made nine birdies on his round at the Spring Mill Course at Philadelphia Country Club to charge in front.
'My first tournament back was the qualifier,' said Beljan. 'I knew if I could get out to a great start, things would get rolling. I didn't hit it that great today, but I made the putts. I made the up-and-downs. Everything fell together. Other than the finals of the Junior, this is one of my best.'
Beljan, a 20-year-old junior at the University of New Mexico, tied for the second-lowest stroke play score in Amateur history.
Ryan Blaum, Andrew Parr, Chandler Cocco, Ryan Yip and Jay Choi are all tied for second place at minus-2.
Seven players got in the clubhouse after shooting 1-under 69, including Mark Strickland and Todd Mitchell, who both played on the more difficult East Course at Merion Golf Club.
Other golfers who sit five off the lead are John Holmes, Seth Cotter, Jesse Barnsley, Matt Savage and Kyle Stanley. Holmes recently played for the U.S. Walker Cup team, which took on Great Britain and Ireland.
The highest ranked golfers from the European Walker cup team were Oliver Fisher and Lloyd Saltman, who both shot even-par 70s.
Fourteen of the 16 players who broke par played at Philadelphia Country Club on Monday. The players will complete on the opposite course on Tuesday, which might allow for more players to break par.
Reigning NCAA champion James Lepp of Canada, who will be a senior this season at the University of Washington, shot a 1-over 71 and is seven shots behind Beljan.
Clay Ogden, the defending U.S. Amateur Public Links Match Play champion posted a 3-over 73, while last year's runner-up at this event Luke List opened with an 80.
The 36-hole stroke play portion of the 105th U.S. Amateur Championship takes place on Monday and Tuesday, while the match-play portion of the event begins on Wednesday. The second and third round matches are scheduled for Thursday, with the quarterfinals slated for Friday. Saturday will feature semifinal play while the 36-hole final will take place on Sunday.