Route 21 is a 5.67-mile (9.12 km) state highway in rural northeastern Connecticut.

Route 21 runs through the rural areas of Putnam as it heads north towards the town of Thompson.

[1] Route 21 is two lanes wide and is classified as a collector road for its entire length.

It was extended north from US 44 to Route 193 in 1957 to provide access to the Quinnatisset Country Club.

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