SAN FRANCISCO  Discerning Internet users know that glowing online reviews of things like books or restaurants cannot always be trusted. But federal regulators are serving notice that if you stand to gain financially from the review you are writing, you should be upfront about it.

The Federal Trade Commission said on Thursday that a California marketing company had settled charges that it engaged in deceptive advertising by having its employees write and post positive reviews of clients’ games in the Apple iTunes Store, without disclosing that they were being paid to do so.

The charges were the first to be brought under a new set of guidelines for Internet endorsements that the agency introduced last year. The guidelines have often been described as rules for bloggers, but they also cover anyone writing reviews on Web sites or promoting products through Facebook or Twitter.

They are meant to impose on the Internet the same kind of truth-in-advertising principles that have long existed offline.