The 2009 world champion's 1m25.936s lap came on medium-compound Pirelli slicks and knocked newly re-signed Force India driver Adrian Sutil off the top spot.
After yet more rain overnight, the session got underway on a damp track and with no serious running taking place inside the first half-hour.
Nico Rosberg was the fastest man in the damp, the German bringing the overall pace down to the 1m30s mark on intermediate tyres.
The Mercedes driver was also right on the pace when the field collectively switched to slick rubber with an hour to go, following the lead set by Williams driver Pastor Maldonado.
The Venezuelan, Adrian Sutil and Fernando Alonso all took turns at the top of the timesheets before Button assumed the top spot with 40 minutes to go.
Sutil wound up 0.215s slower than the Briton in his Force India, while Sauber's Nico Hulkenberg and Rosberg rounded out the top five.
Romain Grosjean, who was sixth fastest, brought the morning running to a premature halt when his Lotus made a 'non-specific stop simulation' with three minutes remaining between Turns 8 and 9.
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