The Prime Minister watched Washington Nationals take on Arizona Diamondbacks during his trip to the US.
Fans were treated to a military flypast and both God Save the King and the Star Spangled Banner played by Royal Marines and US military bands.
Mr Sunak mingled with players before the game and wore a Nationals jacket as he was cheered onto the pitch – although Nationals Park was far from full.
The ceremonial first pitch at the UK-US Friendship Day was thrown by British army veteran Stuart Taylor after Downing Street rejected the idea of Mr Sunak throwing the ball in front of thousands of spectators.
“These sorts of things are pitched to us from time to time but, at this event, the first pitch is going to be thrown by Stuart Taylor, who is the CEO of the Allied Forces Foundation,” the Prime Minister’s spokesman said.
“That, I think we felt, was the most appropriate way of highlighting the breadth and depth of the UK-US relationship, particularly focusing on service personnel and veterans.”
After the pitch, Mr Sunak asked Mr Taylor: “How was that, quite good right?”
The visit came ahead of Mr Sunak’s meeting with Joe Biden on Thursday.
On Wednesday, Mr Sunak announced the UK will host the first global summit on artificial intelligence (AI) safety.
The summit, which will be held in the autumn, will consider the need for international co-ordinated action to mitigate the risks of the emerging technology.
The Prime Minister said: “AI has an incredible potential to transform our lives for the better. But we need to make sure it is developed and used in a way that is safe and secure.
“Time and time again throughout history we have invented paradigm-shifting new technologies and we have harnessed them for the good of humanity.
“That is what we must do again.”
Downing Street said the summit was an opportunity for “like-minded countries” to come together on the issue.
Asked if it was an alliance against China and Russia on AI issues, the Prime Minister’s official spokesman said: “No, it’s about looking at technology that is developing extremely quickly – perhaps faster than even those involved in its creation expected and better understanding the possibilities and potential risks and challenges that poses and how globally we can come up with ways to address that.”
The Prime Minister was also using the trip to announce an increase in the number of scholarships the UK Government funds for students undertaking post-graduate study and research at UK and US universities, especially in science, technology, engineering and maths.
Other subjects on the agenda for the meeting with Mr Biden in the White House include economic co-operation and continuing support for Ukraine, particularly on the provision of air defence.
Mr Sunak could also take the opportunity to push for Defence Secretary Ben Wallace to be handed the job of leading Nato when secretary general Jens Stoltenberg steps down at the end of September.