Penton Hook Lock is the sixth lowest lock of forty four on the non-tidal reaches of the River Thames in England. It faces an island which was until its construction a pronounced meander (a hook) and is on the site of its seasonal cutoff. It is against the left bank marking the church parish medieval border of Laleham and Staines upon Thames in Surrey for many centuries. Until 1965 their county was Middlesex. At 266 ft (81 m) it is the third longest lock on the river.