Is 300 feet by 300 feet an acre?

An acre is 43,560 sqft. That would put the closest dimensions for a square acre at approximately:
208.71 ft x 208.71 ft
(most people I know free hand the math using either 200 ft x 200 ft or 210ft x 210 ft for approximate measurement)

The correct acreage for 300ft x 300ft is:
2.066 acres