In the letter, Sandra Garza, the girlfriend of Brian Sicknick, revealed the Duke of Cambridge paid tribute to her late partner.
Mr Sicknick, suffered two stroked after being assaulted by rioters on January 6, 2021 and died the following day.
In the letter, shown by his partner on CNN, William wrote: “Please forgive me if I am intruding but I wanted to write and let you know how sorry I am about the death of your partner, Brian.”
He then describes how he had watched footage of the riot and the praised Mr Sicknick’s “patriotism”.
“By all accounts, Brian performed valiantly whilst on duty and despite suffering injuries continued to do his utmost to protect those inside,” he added.
“I know that words cannot hope to provide comfort to you at this dreadful time but I wanted to let you know that Brian’s family are very much in my thoughts.”
Over 150 police officers were injured during the riot after a large crowd forced its way into the US Congress as lawmakers gathered to confirm Joe Biden’s election victory.
Ms Garza added during the interview: “Trump does not give two craps about law enforcement or Brian and yet Prince William took the time to honour me and Brian.”
A House Select Committee has been established to assess the incident and was shown footage of the battles which took place between officers and rioters at the time.
Caroline Edwards, who herself suffered a traumatic brain injury from the incident, told lawmakers: “I can just remember my breath catching in my throat, because what I saw was just a war scene. I couldn’t believe my eyes,” Edwards said. “There were officers on the ground, they were bleeding they were throwing up.
“I saw friends on the ground. I was slipping on people’s blood. It was carnage.”