The omnibus, broadcast on ITV2 on June 3 at 8.50am, sparked complaints with a scene featuring three men paying for sex and following 16-year-old Bethany Platt (Lucy Fallon) into a bedroom.
Viewers saw another character, Shona Ramsey (Julia Goulding) being beaten and stamped on by several men.
Chris Harper plays Nathan Curtis in the long-running grooming storyline.
An Ofcom spokesman said: "We're investigating whether this Saturday morning omnibus episode contained scenes that were unsuitable for broadcast at that time."
The watchdog assessed but decided not to investigate 73 complaints about a drug taking scene on the soap and 24 complaints about the grooming storyline on separate episodes in June, broadcast in the evening on ITV.
Ofcom were forced to investigate the Coronation Street omnibus following the Bethany Platt storyline
We concluded that both storylines were clearly signposted
The spokesman said: "We concluded that both storylines were clearly signposted and were the culmination of long-running plots that were likely to have been within the audience's expectations. The drug use was not shown in a positive light, and we took into account Coronation Street's history of exploring distressing social issues."
It also decided not to investigate 92 complaints about comments made in the Big Brother house.
Double bubble: soap stars in other soaps Sun, June 11, 2017
See which of your favourite soap stars have appeared in more than one series.
ITV 1 of 10
Claire King as Kim Tate in Emmerdale and now in Coronation Street as Erica
Many of the complaints were about a "race row".
"We received a number of complaints about the recent series of Big Brother," the spokesman said.
The soap received complaints after the young character became the victim of grooming
An investigation concluded that the ‘storylines were clearly signposted’
"We are satisfied that Channel 5 broadcast clear and appropriate warnings about potentially offensive content, and intervened appropriately in heated exchanges and situations.
"We also took into account the audience's expectations of this well-established reality format, and that the series is shown after the 9pm watershed."