Ricciardo was on a slowdown lap in wet conditions early on in Sochi and was warned of team-mate Lando Norris approaching behind.
After letting the eventual pole-sitter past, Ricciardo then cut back onto the racing line and blocked Aston Martin driver Stroll.
Both drivers progressed from the session despite the incident. Ricciardo will now start from fifth on the grid and Stroll eighth.
A stewards report read: "The driver of car 3 [Ricciardo] was on a slowdown lap and was warned by the McLaren team of the approach of his team-mate.
"Rearwards visibility was affected by the wet conditions and the driver of car 3 was unaware of the approach of car 18 [Stroll] and, in the opinion of the stewards, would have been unable to see its approach in any case until the last moment.
"This was compounded by the spray from car 4 which had just passed car 3.
"The competitor [McLaren F1 Team] admitted it had made a wrong assumption that car 18 was on a slow lap having just completed a fast lap, however in fact it was doing consecutive fast laps due to the improving [but wet] track conditions.
"It, therefore, did not inform the driver of the presence of car 18. The stewards noted that as soon as the driver of car 3 was aware of the presence of car 18, he took immediate action to move off the line.
"The stewards compared this situation to every other impeding case for the past two and a half seasons.
"The stewards believed this bordered on 'unnecessary impeding' hence was a breach of the rules and therefore required a penalty.
'However, considering all the circumstances, it was felt a reprimand was the most appropriate and consistent penalty.
"However, clearly the McLaren team made an error in not accurately monitoring the traffic behind car 3 and therefore an official warning is issued to the competitor and it is noted that a similar failure in future could result in an additional monetary or other penalty to the competitor [and the relevant driver]."
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