The 19-year-old hard-launched her boyfriend on the social media app with a bunch of cute pictures of her and him at the Trojans' Friday night game against Rutgers.

In one of the snaps, Arnold can be seen with a huge grin on her face as the two posed for a pic right on the field. In others, the duo was seen lovingly embracing.

In addition, Arnold also showed the new couple went on a Disneyland date earlier Sunday ... looking all loved up as they roamed around Mickey's Happiest Place On Earth.

Fans of Arnold quickly raced to the comment section of her post to give their stamp of approval ... with even Taylor Lautner weighing in!!

Arnold's love life, of course, has been a popular topic this year ... as many had speculated she and her former dancing partner, Harry Jowsey, were in a fling due to their big chemistry on the show.

But, Arnold said back in September she was single -- although it seems she might have at the very least had eyes on Lyons then.

As for the Lyons -- a 6-foot-4, 235-pound true freshman tight end who's played in five games for USC this year -- his dating history is a bit of a mystery.