The 27-year-old spent last season on loan with the Millers, making 46 appearances as they earned promotion through the League One playoffs.
Towell has been bereft of first-team action at the Amex Stadium, playing in just one league game for the Seagulls since signing from Dundalk in January 2016.
"This loan move is another good opportunity for Richie to go out and play regularly in a highly competitive Championship," Brighton boss Chris Hughton told the official club website.
"He had a very successful time with Rotherham last season and this gives him an opportunity to return to a club he knows well.
"I would like to wish Richie all the very best of luck and we will closely monitor his progress throughout the season."
Towell scored 25 goals in 32 appearances during his final season with Irish club Dundalk ahead of his move to Brighton.