The round amounts to $35m, with the company valued at a total of $485m. Moving forward, the company intends to build licensed sports betting products and hire over 100 new staff.
Underdog has made significant customer acquisitions over the past two years and recently launched a $10m NFL Best Ball tournament.
Underdog President and Chairman Jeremy Levine said: "Since the start of Underdog, we've believed building quality products and putting customers first is a winning recipe.
“We're excited to continue to innovate and build new experiences from the ground up in fantasy, sports betting and adjacent spaces. This funding round gives us the firepower to continue to build and build we will.
"We're thrilled that funds and accounts managed by BlackRock are participating in our Series B as we continue towards our vision.”
BlackRock and Acies have joined existing investors including Mark Cuban, Kevin Durant and ex-CEO of Paddy Power Betfair Breon Corcoran.
Acies Co-Founding Partner Chris Grove said: "Underdog's product is leading to competitive advantages as Underdog's customer acquisition and retention success is best in class.
"Underdog's success is driven by the company's focus on building new, innovative games and delivering amazing customer experiences. As the company moves into licensed sports betting, I believe the unique focus on product and experience will reshape the industry."
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