Niihari (新治郡, Niihari-gun) was a district located in Ibaraki, Japan. The district was dissolved on March 27, 2006.

The district had only one village before dissolution:

• Tamari

Timeline (Heisei Era, 1989-2006)

• January 1, 1992 - The village of Chiyoda was elevated to town status.
• April 1, 1997 - The village of Dejima was elevated to town status and changed the name to Kasumigaura.
• March 28, 2005 - The town of Kasumigaura absorbed the town of Chiyoda to create the city of Kasumigaura.
• October 1, 2005 - The town of Yasato was merged into the expanded city of Ishioka.
• February 20, 2006 - The village of Niihari was merged into the expanded city of Tsuchiura.
• March 27, 2006 - The village of Tamari was merged with the towns of Ogawa and Minori (both from Higashiibaraki District) to create the city of Omitama. Niihari District was dissolved as a result of this merger.