Khan, who is an MLA from minority-dominated Araria, had resigned from LJP on Thursday after it announced its tie-up with BJP for Lok Sabha poll, and joined JD(U) in the presence of its state president Basistha Narayan Singh.
LJP had won three seats in Bihar Assembly polls in 2010 and its two other legislators had quit earlier to join JD(U).
Describing Ramvilas Paswan as an "opportunist" over his joining hands with BJP, Khan said that because he cannot share the same platform with BJP, he took the decision to quit LJP.
On Khan joining JD(U), Singh said it will strengthen the ruling party in Seemanchal region including Araria, Kishanganj, Purnia Madhepura and Supaul.