Tua Tagovailoa is showing serious love for his mom with one of his first big NFL purchases ... surprising her with a Cadillac Escalade for Mother's Day!!!

The 22-year-old Miami Dolphins star rookie revealed the new whip in an emotional video on Sunday ... explaining to Diane, "I know you wanted something that would be able to carry everyone."

Mom's reaction is PRICELESS ... and she can't believe Tua's being serious -- "This is mine?!"