The 20-year-old Haas driver, one of six rookies learning their way on the narrow, bumpy and sinuous Mediterranean street circuit, passed Williams' Carlos Sainz at La Rascasse hairpin after Oscar Piastri had crashed his McLaren at Ste Devote.
The race stewards reported that Bearman had not obeyed the warning lights and dashboard signals about the red flag.
"The driver claimed that he saw the red flags, but decided not to slow down abruptly because he felt that slowing down abruptly would have been more dangerous and that what he did was a safer way of handling the situation," the stewards reported.
Bearman remained upbeat about his prospects.
"We made a good step with the car and I am happy with the balance. There were some good signs from today so let's try and build on them."
Unfortunately, for him, on a difficult day for most of the rookies, his penalty makes it unlikely he will score points on a circuit where overtaking is notoriously difficult.
It was a better day for the RB rookies Liam Lawson and Isack Hadjar who wound up fifth and sixth behind the pace setting Ferrari of Charles Leclerc.
Hadjar survived two major brushes with the barriers on a day of three red flags across the two sessions.
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