Danny Glass and Sophie Rose were riding a scooter on the island of Phuket when he braked suddenly to avoid a car, causing her to fall into the road.
The 41-year-old, who was six months pregnant, was then run over by an 18-wheel lorry – and she died from her injuries.
Charges have also been brought against Nattawoot Kimchue, who was driving the lorry.
Lt Col Sanit Nookong told The Phuket News: “Both men are being held responsible for the death, because both were driving recklessly.
“Right now, the investigation is not yet closed because we are still waiting for the autopsy to be released. Then they will go to court.”
Ms Rose was a popular YouTube vlogger who made videos about breastfeeding, and she has a five-year-old son from a previous relationship.
Glass also created videos about health and wellness on YouTube.
On Monday, he uploaded a clip and said: “I won’t be posting any more videos for a while. I don’t know when I will be back to making videos full time.
“It’s not the right time at the moment. I just need to get my life in order, my life on track and get through this sad death that happened to Sophie.”
A Foreign and Commonwealth Office spokesman told Sky News: “We are in contact with the Thai police and are providing consular support to a British man following a road traffic accident in Phuket.”