The B661 is a short link road in south Huntingdonshire.
It starts at a roundabout on the A1 on the edge of the village of Buckden, from where it heads west along Perry Road, reaching open country very quickly. The B661 has the distinction of connecting to the A1 at that road's first roundabout since Berwick-upon-Tweed: the A1 has 275 miles of uninterrupted free-flow to its north of the B661.
After about a mile the road reaches Graffham Water. The original line of the road was submerged by the building of the reservoir so the road now passes beneath the dam before running to the south of the lake, which is hardly visible. It is just possible to see the old road coming in from the right - but easy to recognise the spot as the road narrows and becomes windier. The road goes through Perry and passes a turn to the reservoir's visitor centre before turning southwest and continuing for another mile or so before ending at a T-junction on the B645 (former A45) in Great Staughton.