Manned Venus Flyby was a 1967–1968 NASA proposal to send three astronauts on a flyby mission to Venus in an Apollo-derived spacecraft in 1973–1974, using a gravity assist to shorten the return journey to Earth.

In the mid-1960s, NASA considered "a three-man flyby of Venus" as part of the Apollo Applications Program, using hardware derived from the Apollo program. Several mission profiles were considered for launch during the 1970s and the 1973 mission appears to be the one that received most serious consideration and is best documented. Launch would have taken place on October 31, 1973, with a Venus flyby on March 3, 1974 and return to Earth on December 1, 1974.

The proposed mission would have used a Saturn V to send three astronauts on a flight past Venus, which would have lasted approximately one year. The S-IVB stage would have been a "wet workshop" similar to the original design of Skylab. In this concept, the interior of the fuel tank would be filled with living quarters and various equipment that did not take up a significant amount of volume. The S-IVB would then be filled with propellants as normal and used to accelerate the craft on its way to Venus. Once the burn was complete, any remaining propellant would be vented to space, and then the larger fuel tank could be used as living space, while the smaller oxygen tank would be used for waste storage.

Only so much equipment could be carried in the hydrogen tank without taking up too much room, while other pieces could not be immersed in liquid hydrogen and survive. This equipment would instead be placed in the interstage area between the S-IVB and the Apollo Command/Service Module (CSM), known as the Spacecraft-LM Adapter (SLA), which normally held the Apollo Lunar Module on lunar missions. To maximize the amount of space available in this area, the Service Propulsion System engine of the CSM would be replaced by two LM Descent Propulsion System engines. These had much smaller engine bells, and would lie within the Service Module instead of extending out the end into the SLA area. This also provided redundancy in the case of a single-engine failure. These engines would have been responsible for both course correction during the flight and the braking burn for Earth re-entry.

Unlike the Apollo lunar missions, the CSM would perform its transposition and docking maneuver with the S-IVB stage before the burn to leave Earth orbit, rather than after. This meant the astronauts would have flown "eyeballs-out", the thrust of the engine pushing them out of their seats rather than into them. This was required because there was only a short window for an abort burn by the CSM to return to Earth after a failure in the S-IVB, so all spacecraft systems needed to be operational and checked out before leaving the parking orbit around Earth to fly to Venus.

Precursors to the Venus flyby would include an initial orbital test flight with an S-IVB "wet workshop" and basic docking adapter, and a year-long test flight taking the S-IVB to a near-geostationary orbit around the Earth.

The mission would have measured:

The mission would have been implemented in a series of two development flights and one production flight, designated as phases A through C.