Mr Musk has also been criticised for other unorthodox behaviour, including appearing on a podcast smoking marijuana and giving a tell-all interview to The New York Times in which he admitted to feeling overwhelmed and unable to sleep over Tesla's ambitious electric auto production targets.
The lawsuit was filed in Los Angeles federal court yesterday and as well as compensation, Unsworth seeks a court order stopping Musk from making further allegations.
The pair first clashed over Mr Musk's offer of a mini-submarine to help with the cave rescue.
But Unsworth said Musk urged in his August 30 email that the BuzzFeed reporter "stop defending child rapists", and called him as "an old, single white guy from England" who moved to Chiang Rai, "renowned for child sex-trafficking", to take a 12-year-old bride.
Musk also said it was suspect that the "British expat guy" was living in Thailand, referring to Thailand's reputation for sex tourism. Unsworth is suing Elon for defamation, and an undisclosed amount of damages. i.e. money. Sure, Musk's going-private tweet certainly caught the DOJ's attention, but the breadth and depth of the probe is pretty much unknown. Mr Unsworth has never engaged in an act of paedophilia.
They also say that Mr Musk "falsely conveyed that [the] United Kingdom citizen.is a paedophile, child rapist, a child sex-trafficker, the husband of a 12-year-old child bride and a liar".
Musk has since backpedaled on this statement. After emailing Musk for comment, the site published his response in which he tells the reporter "stop defending child rapists".
The 47-year-old billionaire, who earlier this month faced investor ire over smoking marijuana on a live web show, has indicated in the past that Tesla's customers may face a longer response time because of a significant increase in vehicle delivery volume in North America.
In late August, Musk suggested via Twitter that it was unusual Unsworth hadn't sued him yet and asked a Twitter user who brought up the matter why he hadn't investigated it.
He recommended that the Thai government seek help from divers in the United Kingdom, and Mr Unsworth called friend and fellow diver Rob Harper.
The boys, aged 11-16, were reached on 2 July by Mr Volanthen and Mr Stanton, according to the lawsuit.
Mr Unsworth did not take part in the dives but had previously explored the cave complex extensively.