The Australian Open champion was not on the entry list when it was revealed on Thursday.
Fellow top-10 star Kei Nishikori was another notable absentee.
However, world number one Andy Murray, Novak Djokovic and nine-time champion Rafael Nadal are all entered.
Federer said last week in Indian Wells that he would make a decision about his claycourt season schedule following the ongoing Miami Open.
However, it would now appear he has no intention of starting it in Monte-Carlo – the tournament is the only Masters 1000 event which is non-mandatory. Technically he could yet request a wild card.
Speaking in California, Federer said: “It's basically (a case of) looking ahead. How can I remain healthy and how can I keep the fire and the motivation for the tournaments that I will be playing?
“What I don't want to do is overplay and just get tired of travelling and tired of just playing tournaments and just entering and, I don't know, just doing people a favour just to be there with no aspirations. That's not why I'm playing.
“I want to play, if people see me, that they see the real me and a guy who is so excited that he's there. So that's a promise I made to myself that if I play tournaments that's how my mindset has to be and will be. So, yeah, we'll see after Miami.”
Federer has never won in Monaco, one of only two Masters venues where he is yet to triumph.
There have been plenty of suggestions that the 35-year-old will play little, if at all, on the clay this season and instead put all his energy into winning a record eighth Wimbledon title.
If he did that, he would skip both the Madrid and Rome Masters tournaments in May – something he is allowed to do due to his long service to the ATP World Tour – as well as the French Open, which starts on May 28.
Federer has already signed up to play in both Stuttgart and Halle prior to Wimbledon.
The Monte-Carlo Masters takes place from April 16-23.
Murray’s fellow Britons Dan Evans and Kyle Edmund have both made the entry cut.
