The Pons Racing driver lapped the British track in 1:41.026, after heavy rain meant that qualifying times were more than 20 seconds slower than yesterday's free practice.
It was Valles' first pole position of the year, boosting this chance of securing one of several high points' finish he needs if he is to stand any hope of beating Robert Kubica to the title.
Jaap Van Lagen will start second, after topping the timesheets in his second session. Because the average of the top times was outside the permitted limits, the drivers from each session will line up in two parallel lines on the grid.
Van Lagen had an eventful session, going off into the gravel twice as he pushed to improve his front-row time.
Monaco race winner Christian Montanari will line up third after being half a second slower than Valles in the first session.
Kubica will start fourth had a slow start to the session and his best time on his final lap as the chequered flag was shown. The Polish driver can theoretically wrap up the title this weekend, should other driver's results go in his favour.
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