Boylan succeeds Scott Skiles, who was fired Christmas Eve after the Bulls had compiled a 9-16 record.
"I feel comfortable giving (Boylan) the interim head coaching position," said John Paxon, the team's executive vice president of basketball operations.
"Jim has paid his dues as an assistant coach and has his own ideas on the way he wants us to play. Many top assistants never get the chance to be a head coach in the NBA and Jim has assured me he will provide the necessary energy and commitment this position requires."
Boylan, 52, is in his fourth season with the Bulls.
He was also an assistant at Cleveland under Mike Fratello, Vancouver under Brian Hill, at Phoenix when Skiles was there and Atlanta under Terry Stotts.