Fletcher joined the Hammers on loan in January last season and went on to score 9 goals in 13 starts.
Last month, Poole agreed terms with Fletcher to sign another contract for 2023/24.
His signing is in recognition of the great progress that he made playing for Hamworthy on loan last season and the Dolphins were happy to loan him back to the Sunseeker International County Ground on a season-long loan.
The 21-year-old was at the AFC Bournemouth academy from under-7 to under 14s.
He signed for Southern Division One South side Wimborne Town in 2018 and scored goals for the development team, and, when he came on as a sub for his first team debut in the Dorset Senior Cup quarter-final he scored one and had an assist for another.
He joined Poole in September 2020.
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