The move is part of a monthlong campaign to dispel popular misconceptions about the religion, the Chicago Tribune reported Wednesday.
Though most people interviewed by the Tribune said they hadn't noticed the ads, Gain Peace is calling the campaign a success.
"We had eight conversions, close to 400 calls and we've had close to 75,000 hits on our Web site in one week," Sabeel Ahmed, director of the Chicago-area group, told the Tribune.
One of the converts is Moses Robinson who saw a CTA bus with the group's ad while on a break from work. Robinson said he converted to Islam after calling the number on the side of the bus and meeting with Gain Peace.
The campaign is doing so well, Gain Peace said it plans to continue the ads for an additional six weeks, the Tribune reported.