A still-unreleased police report obtained by the Associated Press quoted various sources that stated Joseph had several links to the attack, with at least one identifying him among the leaders.
The sources said Mr Joseph paid in cash for rental cars used by the attackers and had met with other suspects ahead of the killing, including Christian Emmanuel Sanon.
Mr Sanon is a well-known Haitian businessman and evangelical pastor who had expressed a desire to lead his country.
Associates have suggested that the true masterminds of the assassination duped Mr Sanon.
He was arrested shortly after the killing.
The report also stated that the former senator introduced other suspects to Joseph Badio, an alleged leader of the plot.
Mr Badio had previously worked for Haiti’s Ministry of Justice and the government’s anti-corruption unit before he was fired.
It also said phone records show James Solages, a Haitian-American arrested in the case, had a WhatsApp conversation with Joseph regarding preparations for the mission.
And it said that Mr Solage told authorities that Mr Joseph, Mr Badio and Rodolphe Jaar were among those appointed leaders of the operation.
M Badio remains a fugitive, while Dominican officials said Mr Jaar was arrested there at the request of US authorities.
Among those celebrating the arrest was Claude Joseph, Haiti’s former minister of foreign affairs who briefly served as interim prime minister following Mr Moïse’s killing.
“The arrest of John Joël Joseph shows that there will be no hiding place for those who are directly or indirectly involved in the assassination,” he wrote.
The 122-page police report said authorities visited at least three homes from July 10 to July 21 in efforts to track down John Joël Joseph, finding nothing except for four 12-gauge rifles, ammunition and firearms accessories in the first house that was under his name.
Long list of suspects
More than 40 people, including 18 former Colombian soldiers, have been arrested in the killing of Mr Moïse.
John Joël Joseph is the second suspect to be arrested in Jamaica.
In late October, Jamaican authorities arrested former Colombian soldier Mario Antonio Palacios Palacios, whom US officials had interviewed while he was in hiding.
Mr Palacios was recently extradited to the US and was awaiting a court hearing after being charged with conspiracy to commit murder or kidnapping outside the United States.
He was also charged with providing material support resulting in death, knowing or intending that such material support would be used to prepare for or carry out the conspiracy to kill or kidnap.
Colombian government officials have said that the majority of former soldiers were duped and did not know about the real mission.
The soldiers, who remain in prison in Haiti, have accused authorities of torture.
At the same time, the Colombian government recently said the country’s consul in Haiti was threatened after trying to provide humanitarian assistance.