Secret (traditional Chinese: 不能說的·秘密; simplified Chinese: 不能说的·秘密; pinyin: Bùnéng shuō de·mìmì; lit. 'The Secret That Cannot Be Told') is a 2007 musical romantic film. A Taiwanese-Hong Kong co-production, it is the directorial debut feature film of Taiwanese musician Jay Chou, who also stars as the male lead and co-wrote the film. In addition to himself, the film also stars Gwei Lun-mei and Anthony Wong. Filming was conducted at Tamkang High School, New Taipei City, Taiwan, which is also the alma mater of Jay Chou. The budget was about NT$65 million.

In 2007, it received six nominations at the 44th Golden Horse Awards and won Outstanding Taiwanese Film of the Year, Best Original Song for "Secret" ("不能說的祕密") and Best Visual Effects. It was also nominated for Best Asian Film at the 27th Hong Kong Film Awards in 2008. In 2016, an adapted brand new stage musical with same name was jointly produced by US Broadway and China Broadway.

Piano prodigy Ye Xianglun (Jay Chou) lives with his father (Anthony Wong Chau-sang). In 1999, Xianglun transfers into the famous music school, Tamkang (Danjiang) Secondary School, where his father is a teacher. His classmate Qingyi (Alice Tzeng) gives him a campus tour, noting that the piano building will be torn down on graduation day. In the building, he hears a beautiful, mysterious melody ("Secret"), leading him to Lu Xiaoyu (Gwei Lun-mei), another piano student. After class, Xianglun asks Xiaoyu about the song; she tells him it is a secret that cannot be told.

Xianglun and Xiaoyu spend time together in the practice rooms or around town. They share their first kiss after he gives her a rare music score that he won for her at a school piano battle. When Xianglun tells Xiaoyu about the demolition of the piano building on graduation day, she plays him the mysterious song "Secret", telling him never to play it on the piano room's oldest piano. Xiaoyu then suddenly disappears and skips class for days. Xiaoyu also begins to develop an undercurrent of jealousy as Qingyi tries to get closer with Xianglun.

In class a few days later, Xianglun passes a note to Xiaoyu asking her to meet him at the practice rooms. However, Qingyi shows up instead, and Xianglun accidentally kisses her since his eyes are closed. He realizes his mistake when the janitor loudly says hello to Xiaoyu outside the door; Xianglun rushes out, but she is already out of sight. He goes to Xiaoyu's home to clear things up, but her mother says she is sick and had dropped out of school a long time ago. She tells Xianglun not to look for her daughter again.

Five months later, graduation day had arrived, with Xianglun set to perform on stage at the graduation ceremony. Qingyi lends him her bracelet to wish him luck. Halfway through the performance, Xianglun sees Xiaoyu by the door, and they share a smile before she quickly leaves. Xianglun abandons the performance and runs outside, chasing down Xiaoyu and embracing her, telling her to wait for him. However, before he goes back, she sees Qingyi's bracelet on his wrist.

When Xianglun goes looking for Xiaoyu after the ceremony a few minutes later, he cannot find her anywhere. Upon asking a few classmates if they had seen the girl he was always with, they are confused and reply that he was always alone. Xianglun begins to have flashbacks to all the times they hung out together, but in those memories, she is missing and he is alone. Desperate for answers, he races to Xiaoyu's house. She is missing, but Xiaoyu's mother gives him a photo of Xiaoyu with his father.

Xianglun rushes home and asks his father about her. The film cuts to a scene with Xiaoyu, in 1979 (20 years ago), speaking with Xianglun's father (her teacher at the time, at the same school). She confesses that she had found a score hidden in the oldest piano on campus, called "Secret", and when she played it on the piano, she had traveled forward in time 20 years and met Xianglun. However, she explains that during her time travels, she can only be seen by the first person she sees, and she doesn't always succeed in seeing Xianglun first, so it appears as though she is missing most days. However, she is now depressed since she had caught him kissing another girl, then saw that girl's bracelet on his wrist. Since she never wants to see him again, she gives the score of "Secret" to Xianglun's father.