The 27-year-old German arrives at Griffin Park on a one-year contract with an option for a further two seasons after failing to become a first-team regular at Hull.
Proschwitz joined the KC Stadium outfit from German club Paderborn in 2012, but found opportunities scarce, making just five league starts for Steve Bruce's men.
He spent the second half of last season on loan at Barnsley, scoring four goals in 14 Championship appearances, and Brentford boss Mark Warburton is looking forward to seeing his new man in action.
"Nick is a very experienced player," he said. "He put in some fantastic performances in Germany before his move to Hull.
"He is an aerial threat and has very good technical ability, and we think he will bring outstanding quality to the squad."
The forward, who had previously played in Liechtenstein and Switzerland as well as his native Germany, scored seven goals for Hull in all competitions.
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