Together AI has closed an $800 million Series C at an $8.3 billion post-money valuation, led by Aramco Ventures with participation from Vista Equity Partners, General Catalyst, Emergence Capital, NVIDIA, March Capital, Pegatron, and S Ventures (SentinelOne), among others. The round is one of the largest yet for a company positioned squarely in the open source AI infrastructure layer, and it lands at a moment when the economics of running AI at scale have become a defining concern for anyone building on top of frontier models.