Paul Pierce Blasted by Baby Mama Princess Santiago Over Father's Day Snub

Paul Pierce's Father's Day came with some serious drama ... 'cause Princess Santiago says the basketball Hall of Famer forgot all about their baby boy ... something she claims is becoming a theme.

Princess unloaded on Paul in a series of Instagram posts Sunday ... blasting the former Celtics star for celebrating Father's Day online while allegedly failing to spend time with their infant son, King.

Her posts centered around a Father's Day tribute Paul shared featuring four of his other children ... with Santiago claiming King -- who is approaching 5 months old -- was notably absent from Paul's plans despite being the first Father's Day since King's birth.

Princess also alleged Paul has only seen the child twice since he was born ... and she later doubled down in a since-deleted comment under one of the posts.

The social media outburst marks the latest chapter in what's been a contentious situation between the Paul and Princess.

As TMZ Sports previously reported, Princess filed a paternity lawsuit against Paul in January while pregnant with King, seeking DNA testing to establish paternity. She later asked for sole custody of the child and nearly $30K per month in child support.

Paul acknowledged he was King's biological father in court filings in April and requested joint legal and physical custody.