The group spokeswoman said the ad was published on Thursday by the Palestinian newspaper Al-Quds, which is circulated in the West Bank, but the Gaza-based daily 'Palestine' refused to print it.
Shalit was captured by Palestinian terrorists during a cross-border raid on an IDF base near Gaza exactly three years ago.
"We wanted to publish the ad in the Gaza Strip as well, but we were unsuccessful," the spokeswoman told Ynet.
"They ('Palestine' newspaper staff) did not give us a reason for the refusal, but we assume it's because the issue is a complex one (in the Hamas-ruled territory). The press in Gaza is apparently not so free. The ad was published in Al-Quds, and we hope the residents of Gaza will read it there."
The B'Tselem spokeswoman continued to say that "the Red Cross is forbidden from visiting Shalit, and his family hasn't received a letter or another sign of life from him. No one knows what condition he's in or in what conditions he is being held.
"B'Tselem is demanding the release of Shalit and that of all the Palestinian prisoners who are being held illegally (by Israel)," she said.
According to a poll conducted by the Rafi Smith Institute for Ynet, half of Israelis believe Shalit will be freed within a year.
The study also found that 69% of adult Jewish Israelis support the release of Palestinian prisoners who have been directly involved in terror attacks in exchange for Shalit.
Twenty-three percent said they opposed the release of prisoners 'with blood on their hands,' and 8% did not disclose their opinion on the matter.
Daniel Edelson contributed to the report