Lalmonirhat Sadar () is an upazila of Lalmonirhat District in the Division of Rangpur, Bangladesh. Its headquarters are in Lalmonirhat.
Lalmonirhat Sadar is located at . It has a total area of 263.83 km<sup>2</sup>. Teesta River flows inside this upazila.
According to the 2022 Bangladeshi census, Lalmonirhat Sadar Upazila had 91,859 households and a population of 373,470. 9.12% of the population were under 5 years of age. Lalmonirhat Sadar had a literacy rate (age 7 and over) of 69.11%: 72.82% for males and 65.46% for females, and a sex ratio of 99.11 males for every 100 females. 91,371 (24.47%) lived in urban areas.
According to the 2011 Census of Bangladesh, Lalmonirhat Sadar Upazila had 79,147 households and a population of 333,166. 78,228 (23.48%) were under 10 years of age. Lalmonirhat Sadar had a literacy rate (age 7 and over) of 47.37%, compared to the national average of 51.8%, and a sex ratio of 998 females per 1000 males. 60,322 (18.11%) lived in urban areas.
As of the 1991 Bangladesh census, Lalmonirhat Sadar has a population of 260876. Males constitute are 51.52% of the population, and females 48.48%. This Upazila's eighteen up population is 131281. Lalmonirhat Sadar has an average literacy rate of 27.7% (7+ years), and the national average of 32.4% literate.
Lalmonirhat Sadar Upazila is divided into Lalmonirhat Municipality and nine union parishads: Barobari, Gokunda, Harati, Khuniagachh, Kulaghat, Mogolhat, Mohendranagar, Panchagram, and Rajpur. The union parishads are subdivided into 117 mauzas and 173 villages.
Lalmonirhat Municipality is subdivided into 9 wards and 64 mahallas.
There are 19 colleges in the upazila. They include honors level Lalmonirhat Government College.