The actress's rep announced the sad news Tuesday, saying ... "Susan died peacefully on May 2 surrounded by loved ones." No other details surrounding her death were provided, including a possible cause.

Susan is most famous for playing Patty Simcox in the 1978 hit musical film -- leaving a mark as Sandy's girly-girl pal ... who had a striking appearance thanks to her big glasses.

She was heavily featured in "Grease" ... showing up for just about all the big musical sequences ... including in "Summer Nights," where she was cozied up next to ONJ.

Susan also had a memorable moment at the school dance scene ... where her character was putting on a show -- only to be taunted and hassled by the boys, who lifted up her dress.

While "Grease" was certainly SB's biggest claim to fame ... she'd actually acted in a handful of other movies and TV shows over the years as well -- including roles in "The Love Boat," "Starsky and Hutch," "The Amazing Howard Hughes," "Switch," "Deadly Blessing," and more.

After getting out of acting in the '80s, Susan continued directing and teaching theater locally, as well as dance. She's survived by her son, her daughter, her grandchildren and other family.

Susan was 72.