The National Space Institute was a space advocacy group established by Wernher von Braun in 1974.

In its earliest stages it was known as the National Space Association, and it changed its name to National Space Society shortly before its 1986 merger with the L5 Society. The merged organization (see NSS article) has continued to use the NSS name after a membership vote on whether or not to change it to Space Frontier Society.

Von Braun was the first president, but quickly became chairman with journalist Hugh Downs as president. Charles C. Hewitt was the first executive director.

After some years of publishing a newsletter that became known as Insight, the Institute entered into an arrangement with the publishers of Omni magazine whereby all members except life members would receive that magazine as part of their membership. Omni editor Ben Bova joined the NSI board, then became vice president and finally succeeded Downs as president. He continued as president after the Omni arrangement was discontinued and arrangement was made with Palmer Publications to make Space World the NSI member magazine.

Under Bova and executive director Glen P. Wilson the arrangements for merger with L5 were concluded, and the name change to NSS announced in advance of the merger, with vague explanations to the members that the name had to be changed because the name had to be changed. Since the merger it has been claimed that the name change took place upon the merger, but the historical record to the contrary is clear.