Formula One championship leader Hamilton, winner of the last four races, lapped the winding street circuit with a best time of one minute 16.758 seconds on a bright and sunny morning in the principality.
Ricciardo was a mere 0.050 off his pace with Nico Rosberg, last year's winner from pole position, 0.116 slower than Hamilton, who set his time five minutes from the end on supersoft tyres.
Hamilton leads Rosberg, his main title rival, by three points after four successive one-two finishes in the last five races.
The Briton has never qualified on pole position before at Monaco, although he won the race in 2008 with McLaren on his way to the title, but looks favourite to do so in later qualifying.
Red Bull's quadruple world champion Sebastian Vettel was fourth on the timesheets ahead of what will be his 100th start for the team.
Ferrari's Fernando Alonso, whose team have now gone a year without a win, was fifth fastest.
(Reporting by Alan Baldwin, editing by Patrick Johnston)
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