Juno II was an American space launch vehicle used during the late 1950s and early 1960s. It was derived from the Jupiter missile, which was used as the first stage.

Solid-fueled rocket motors derived from the MGM-29 Sergeant were used as upper stages, eleven for the second stage, three for the third stage, and one for the fourth stage, the same configuration as used for the upper stages of the smaller Juno I launch vehicle. On some launches to low Earth orbit the fourth stage was not flown, allowing the launch vehicle to carry an additional nine kilograms of payload. Development of the Juno II was extremely fast due to being completely built from existing hardware. The project began in early 1958, and the first vehicle flew at the end of the year. Chrysler was responsible for the overall contract, while Rocketdyne handled the first stage propulsion and Jet Propulsion Laboratory handled the upper stage propulsion. The first three Juno IIs were converted Jupiter missiles, however all remaining boosters were built as Juno IIs from the beginning.

The main differences between the Juno II and Jupiter were stretched propellant tanks for increased burn time (the first stage burn time was approximately 20 seconds longer than on the Jupiter), a reinforced structure to support the added weight of upper stages, and the inertial guidance system replaced with a radio ground guidance package, which was moved to the upper stages.

The Juno II was used for ten satellite launches, of which six failed. It launched Pioneer 3, Pioneer 4, Explorer 7, Explorer 8, and Explorer 11 from Cape Canaveral Launch Complex 5 and Launch Complex 26B.

The first launch of a Juno II, Pioneer 3 on 6 December 1958, suffered a premature first-stage cutoff, preventing the upper stages from achieving sufficient velocity. Pioneer 3 could not escape Earth orbit, but transmitted data for some 40 hours before reentering the atmosphere. A malfunction in a propellant depletion circuit was found to be the cause of the failure, although the exact nature of it could not be determined. The circuit was redesigned afterwards.

Pioneer 4 launched successfully on 3 March 1959, making for the only first-generation U.S. lunar probe to accomplish all of its mission goals, as well as the sole successful U.S. lunar probe until 1964. After Pioneer 4, NASA shifted their lunar efforts to the bigger Atlas-Able booster and decided instead to utilize the Juno II for Earth orbital launches. By removing the fourth stage, the payload capacity was nearly doubled.

The attempted launch of an Explorer satellite on 16 July 1959 failed dramatically when the Juno II lost control almost immediately at liftoff, performing a cartwheel before the range safety officer sent the destruct command. The almost fully fueled booster crashed a few hundred feet from the pad, blockhouse crews watching in stunned surprise at the upper stage motors burning on the ground. Cause of the mishap was quickly traced to a short between two diodes in a power inverter, which cut off power to the guidance system at liftoff and caused the Juno's engine to gimbal to full stop, flipping the vehicle onto its side before Range Safety action was taken. To prevent a recurrence of this failure mode, improved coatings were used on the circuit boards in the booster.

On 15 August 1959, the next Juno II was flown, carrying the Beacon satellite. While first-stage performance was nominal, the upper stages malfunctioned. One intended experiment on this mission was the ejection of four flares stowed in the interstage section, which would be tracked and photographed during the launch. However, things went awry when the flare ejection failed to take place on schedule. The control system also malfunctioned and drove the upper stages into the Atlantic Ocean instead of orbit. It was concluded that one of the flares deployed inside the interstage section instead of outside like it was intended to, causing the guidance compartment to depressurize and cause loss of vehicle control.