The show, which airs live for two and a half hours every Sunday, is presented by Tim Lovejoy and Simon Rimmer.
The episode on August 12 featured an interview with three British Military Fitness (BMF) representatives on the project, which provides outdoor fitness classes led by former members of the armed forces.
Rimmer and Lovejoy wore T-shirts bearing the BMF logo during the item, and also engaged in some of the exercises.
But a viewer complained that the item appeared to be an advertisement for BMF as it contained details on pricing and membership packages, a free trial and a special offer for Sunday Brunch viewers.
Channel 4 told Ofcom that neither it nor the programme makers had received any payment from BMF for featuring on Sunday Brunch.
Channel 4 also acknowledged that Lovejoy and Rimmer wearing BMF T-shirts during the segment gave "undue prominence beyond what may be editorially justified".
In its judgment, Ofcom said that there was "insufficient editorial justification" for the extent and number of references to BMF in the item.
"We were concerned, in particular, by the way in which the discussion gave the BMF representatives the opportunity to convey in a positive light details about their company's growth, success, and the prices charged to consumers, with relatively less focus on the exercises involved," Ofcom said.
It added: "Ofcom also considered that the overall effect of the item was to promote BMF. The specific references to the success of BMF, the cost of its classes, the no-obligation free trial and the special offer on the Sunday Brunch programme website, clearly promoted the company, in breach of Rule 9.4 of the Code."
Ofcom noted Channel 4's efforts to improve its compliance procedures, but warned the broadcaster to take a "more thorough approach" to clearing items appearing on Sunday Brunch in the future.