The U.S. government agency that developed TCP/IP was DARPA, the Defense Advanced Research Projects Agency.
DARPA funded and coordinated much of the research that led to modern internetworking. In the 1970s, researchers Robert Kahn and Vint Cerf worked on protocols that could connect separate packet-switched networks, allowing them to communicate even when their internal technologies differed. Their work became the foundation of TCP/IP.
TCP/IP is a protocol suite rather than one single protocol. TCP originally handled reliable delivery and sequencing, while IP handled addressing and forwarding packets between networks. The original Transmission Control Program was later split into the separate Transmission Control Protocol and Internet Protocol.
DARPA was originally called ARPA, or the Advanced Research Projects Agency. The agency added “Defense” to its name in 1972, changed back to ARPA in 1993, and became DARPA again in 1996. CERN created the Web, NASA helped develop networking for space communications, and NSF funded major network expansion, but DARPA is the canonical answer here.