It has been a "huge honour" to head the party, Mr. Beattie said in a statement.
"It is now clear that some believe the momentum needed to keep the Ulster Unionist Party moving in the right direction cannot come from me," he stated.
"Irreconcilable differences between myself and Party Officers combined with the inability to influence and shape the party going forward means that I can no longer remain the Party Leader."
He declared that he will step aside and let the party start the process of choosing a new leader right now.
Mr. Beattie conceded that not everyone in the party shared his vision for the organisation or the course he set for it.
"I hope they will see that our future can only be secured in the long run by an inclusive Ulster Unionist Party that spreads a constructive message. I hope the future leader has unrestricted authority."
This appears to be a result of internal conflicts between Mr. Beattie and other party leaders.
Doug Beattie resignation in full... @irish_news pic.twitter.com/05NbzoRUAP
— John Manley - Irish News Pol Corr (@politicsIN) August 19, 2024
In 2021, Mr. Beattie, a distinguished veteran of the British Army, was chosen as the UUP leader.
When Steve Aiken resigned, the member of the Upper Bann Assembly took over.
A 34-year veteran of the military who received the Military Cross for valour in battle, Mr. Beattie came to the position vowing to forward a "progressive and unifying" agenda.
The UUP secured a Westminster seat for the first time since 2017 in a generally favourable general election result earlier this summer, which sparked concerns about his future.