The organization announced the massive, 22,209-fan turnout at Lucas Oil Stadium ... which edged out the previous attendance record set on the first day of the event -- 20,689 spectators.

Those fans had the privilege of watching the GOAT of women's swimming do her thing. The 27-year-old dominated, per usual ... crushing her competition in the 1500-meter swim. Katie finished with a time of 15:37.35 ... and second place finished 20 seconds later!!

While beating the competition by that much time is already an impressive feat for any swimmer, Ledecky told NBC Sports after the race she thought she could have done better!!

"I was pretty excited coming into tonight," she said. "I would have loved to have been a little faster, but I’ll take it. I’ll be better in a few weeks."

Fun fact -- Ledecky holds the 19 fastest times EVER in the event ... and Wednesday night's performance was her 16th-best time. So yeah, she actually CAN do better, which is crazy.

It marked her third win at the trials ... as she also took home first in the 200-meter and the 400-meter freestyle swims.

The GOAT has one more event to take place in -- the 800-meter freestyle -- and we take it there will be plenty of fans there to get a glimpse of some more greatness from the rest of the field as well.