Lalejin District () is in Bahar County, Hamadan province, Iran. Its capital is the city of Lalejin.
After the 2011 National Census, the village of Mohajeran was elevated to the status of a city.
At the time of the 2006 census, the district's population was 44,568 in 10,635 households. The following census in 2011 counted 45,419 people in 12,640 households. The 2016 census measured the population of the district as 41,383 inhabitants in 12,774 households.