In the Star Trek TOS episode "Savage Curtain", Scotty is shown wearing a tartan and a sporran as part of his dress uniform, as seen below:
Is this an actual clan tartan, and if so, to which clan does it belong?
This Wired article says that the tartan is one of the many Clan Scott designs.
It looks like pieces of the actual tartan were sold and autographed as memorabilia, with a piece recently being auctioned off in 2010, on this online auction site. The page includes a close up of the tartan that allows a better look at the pattern:
From this online registry of Scottish clans, this indeed appears to match the "Clan Scott Black & White Ancient" pattern:
Yes. It's one of the ancient Scott Clan Tartans
According to Memory Alpha (referencing The World of Star Trek) the producers were keen to ensure that they had the right one and asked a Scottish connection to procure the correct tartan for Mr Scott.
The idea for a kilt to be included as part of Scott's dress uniform was thought up by actor James Doohan. He later recalled, "They [the producers] immediately grabbed onto the idea. They sent to Scotland for the Scott tartan." (The World of Star Trek, 3rd ed., p. 94)
It may interest you to learn that actor James Doohan regularly wore that particular tartan at events.