Dobson was Arsenals U-18 captain last season, and featured regularly in the clubs U-19 UEFA Youth League.
It is believed that once West Ham shown a strong interest in acquiring his services to continue his development, Dobson requested the move, with the Gunners then reluctantly letting him go.
Dobson was delighted with the move, telling whufc.com ”I am over the moon to have signed and have been a West Ham fan since I was five-years-of-age and this is a surreal moment. Putting pen to paper is a dream come true for me.
“I had a great ten years with Arsenal and they helped me as a footballer and as a person. But the time is right to move and I cannot wait to get started.”