In a letter of support to al-Hajj, a copy of which was released by the Committee to Protect Journalists (CPJ) in New York on Thursday, Alan Johnston also thanked al-Hajj for his appeal to the Gaza kidnappers to release him earlier this year.
Johnston disappeared on March 12 while driving his car in the Gaza Strip and was held for nearly four months.
Journalists around the world repeatedly called for the BBC reporter's release.
Al-Hajj made a public appeal to the Gaza kidnappers in March, saying: "While the United States has kidnapped me and held me for years on end, this is not a lesson that Muslims should copy."
Speaking to Al Jazeera on Thursday, Johnston said he had been moved by al-Hajj adding his voice to the campaign for his release, given the enormous problems al-Hajj has faced in the US detention centre.
He said: "I understand that Mr al-Hajj is demanding the right to answer the accusations that have been made against him now for some five years in Guantanamo Bay and of course, I would always support any prisoner's right to a fair trial."
Since being detained, al-Hajj has never been charged.
Clive Stafford Smith, al-Hajj's lawyer, has called accusations of ties to al-Qaeda baseless and reported that US interrogators focused almost exclusively on obtaining intelligence on Al Jazeera.
Smith has emphasized al-Hajj's deteriorating health in detention, saying he has staged a hunger strike and was in declining physical and mental health, having lost nearly 18kg.
Ould Sidi Mohammed, a Mauritanian recently released from Guantanamo, on Wednesday said the Al Jazeera cameraman was losing weight continuously, suffering from a kidney infection and getting inadequate medical treatment.
Hopes that al-Hajj would be released at the end of August on condition that he remain in his country of origin, Sudan, have proved unfounded.
There are about 340 remaining detainees at the Guantanamo prison.
The first prisoners arrived nearly six years ago after the United States began what George Bush, the US president, has called a "war on terrorism" in response to the attacks on September 11, 2001.