Secret "X" is a game based on Tic-Tac-Toe but without the Os, only Xs.
- The centerpiece of Secret "X" is a large tic-tac-toe board. The middle column of the board contains a secret X hidden in one of its three boxes. To win a prize, the contestant must get three Xs in a row horizontally or diagonally.
- The contestant is given a free X to place anywhere they want in the left or right columns of the board. The contestant is then shown two small prizes one at a time, each with two prices displayed. If the contestant picks the correct price, they win the small prize and earn an additional X, which they then place on the board. Once both prizes have been played, the middle column of the board flips over, revealing the location of the secret X. If the secret X completes a string of three Xs in a row, the contestant wins. A winning row of three must include the secret X, and contestants are not allowed to place all three of their Xs on the same side of the board to create a vertical row (although this is no longer explicitly stated).
- If the contestant earns no additional Xs, the game is automatically over. However, even if they earn both additional Xs, the contestant can still only form a row with two of the three secret squares and is not assured victory.
- Contestants are often discouraged if they attempt to place an X in the middle row, because such a placement can only win with one possible secret X position - the center. The same can be accomplished by placing Xs in vertically-opposite corners; this also allows a third X (if won) to also complete the top or bottom horizontal row, which is not possible with the middle-row placement.
- Secret "X" premiered on September 14, 1977 (#2473D) and was created by former producer Kathy Greco, who was then a production assistant on the show. It premiered on the same day as the now-famous incident involving contestant Yolanda Bowersley's tube top falling when she was called to "Come on Down". On its first playing, it was lost right away with no additional "X"s; though Bob revealed the center column to find out where the Secret "X" was, after learning that he had to push a button to reveal not knowing it. On its second playing, the game received its first win.
- When the game made its debut in 1977, there were no question marks in the center column; they were added presumably on October 26, 1977 (#2533D). On February 12, 1981 (#3994D) the white parts of the setup became yellow. On October 10, 1986 (#6215D) the clear background of the Secret "X" logo changed to yellow, the white border turned red and the red base became black, also the small prize podiums was also revised completely. The black stripes in the corners were removed on November 4, 2004 (#3034K). The font on the choice prices and the actual prices changed to Kingpin on November 27, 2007 (4102K, aired out of order on November 5).
- One notable playing on March 7, 1995 (#9492D) featured some technical difficulties. The machine was stuck as Bob pushed the button, and he called out to Roger Dobkowitz for help, at first trying to get into the black tarp at the back of the board and crank the middle column manually, but did not help. As a result, Bob and Roger turned the board around and looked behind the black tarp at the back of the board to find out where the Secret "X" was. It was on the bottom, and the contestant lost.
- On another playing, from December 4, 1998, an error occurred in which the prices - both the choices and the ARP - were reversed on both prizes. After guessing the first small prize wrong and the second one right - and trying to get three in a row on top - everyone realizes the error on pricing the small prizes, and so the only thing to do was to award him the prize, but Bob checked to reveal the center column anyway, and he would not have won since the Secret "X" was in the middle, but he might have put his second "X" on the bottom right as Bob said it.
- Another notable playing, from April 27, 1999 (#1102K), featured a contestant who was clueless about the game and tried to get three in a row on the left column. She first placed her free “X” in the middle left, then placed her second on the top left after getting her first small prize correctly. But, she guessed the second prize incorrectly, resulting in an instant loss, but Bob pushed the button to reveal the center column, and the Secret “X” was in the middle.
- On February 14, 2014 (#6615K), a trip to Cancun for 22 people worth $33,000 was offered and won.
- In the March 30, 2016 (#7473K) playing, the contestant put his free X in the middle of the right column, effectively meaning that he could only win if the X was in the middle. He won the 2nd X, and then Drew went ahead and played out the game for the 3rd X, even though, at that point, it didn't matter where the contestant put it. The X was indeed in the middle, so the contestant's unorthodox strategy worked.
- On May 5, 2016 (#7524K), contestant Erica guessed both small prizes wrong. She only had the first X, so she had no chance of winning. The same happened to contestant Floriane on November 24, 2015 (#7302K), and to contestant Toni on February 17, 2017 (#7815K).
- On October 28, 2016 (#7665K), Secret "X" was played for a cash awarded prize of $15,000, in honor of Price's Big Money Week, and was won.
- On November 15, 2016 (#7692K), contestant Ben had correctly guessed both small prizes, but still lost the game, which spoiled a perfect show.
- The most number of times this game was played in any season was 36.
- A vertical tic-tac-toe counts as a win on the 1990 computer games released for the PC and Commodore 64, despite it being against the rules on the actual show.