Facebook announced on Jan. 7 that Trump was locked out of his accounts on Facebook and Instagram indefinitely in the wake of the Jan. 6 Capitol siege, over concerns that his posts were inciting violence.
In a separate statement released on Wednesday, Trump again baselessly claimed there was fraud in the 2020 election.
Within six months, "Facebook must reexamine the arbitrary penalty it imposed on January 7 and decide the appropriate penalty," the board said Wednesday. "This penalty must be based on the gravity of the violation and the prospect of future harm. It must also be consistent with Facebook's rules for severe violations, which must, in turn, be clear, necessary and proportionate."
"It is not permissible for Facebook to keep a user off the platform for an undefined period, with no criteria for when or whether the account will be restored," the board said.
The board claimed that Facebook gave Trump's account "a vague, standardless penalty" and then tried to "avoid its responsibilities" by sending the "case to the Board to resolve."
"If Facebook decides to restore Mr. Trump's accounts, the company should apply its rules to that decision, including any changes made in response to the Board's policy recommendations below," the board's ruling said. "In this scenario, Facebook must address any further violations promptly and in accordance with its established content policies."
As part of the board's examination, Trump was allowed to provide a statement, which was submitted by the American Center for Law and Justice on his behalf.
The statement denies that Trump's comments lead to the Capitol siege. "It is stunningly clear that in his speech there was no call to insurrection, no incitement to violence, and no threat to public safety in any manner," and that there is a "total absence of any serious linkage between the Trump speech and the Capitol building incursion."
The statement also claimed that "all genuine Trump political supporters were law-abiding" and that the incursion was "certainly influenced, and most probably ignited by outside forces."
In response to the board's ruling, Facebook said, "We will now consider the board's decision and determine an action that is clear and proportionate. In the meantime, Mr. Trump's accounts remain suspended."
"The board also made a number of recommendations on how we should improve our policies," Facebook said. "While these recommendations are not binding, we actively sought the board's views on our policies around political figures and will carefully review its recommendations."
Trump's supporters and critics are reacting to the board's decision.
House Minority Leader Kevin McCarthy tweeted, "If they can ban President Trump, all conservative voices could be next."
"For every liberal celebrating Trump’s social media ban, if the Big Tech oligarchs can muzzle the former President, what’s to stop them from silencing you?" Cruz tweeted.
Rep. Adam Schiff, D-Calif., responded, "There's no Constitutional protection for using social media to incite an insurrection. Trump is willing to do anything for himself no matter the danger to our country. His big lies have cost America dearly. And until he stops, Facebook must ban him. Which is to say, forever."
Trump told ABC News recently that the written statements he's been issuing during his social media ban are "so much more elegant than Twitter."
ABC News' Quinn Scanlan contributed to this report.