Niwa Nagahide (丹羽 長秀?, October 16, 1535 - May 15, 1585), also known as Gorōzaemon (五郎左衛門), was a Japanese samurai of the Sengoku through Azuchi-Momoyama periods of the 16th century. He served as a retainer to the Oda clan, and was eventually a daimyo in his own right.
Nagahide was born in what is now Nishi-ku, Nagoya(situated in contemporary Aichi District, Owari Province), From his youth, Nagahide served Oda Nobunaga and became one of his senior retainers, going on to fight in the Oda clan's major battles, such as the Battle of Nagashino. He was also an effective governor tasked with the construction of Azuchi Castle, among many of other deeds. The extent of Nobunaga's trust can be seen by the fact that Nagahide married Nobunaga's adopted daughter and his son, Niwa Nagashige, married the fourth daughter of Nobunaga.
These services let Nagahide rule over Wakasa Province and Sawayama Castle in Ōmi Province. In 1581, in a military parade held at Kyoto before the eyes of the Emperor as well as foreign missionaries, Nagahide was given the honour of leading the procession.
On 1582, as Oda Nobutaka's second in command, Nobunaga had Nagahide launch a campaign on Shikoku but before he made any progress, Nobunaga was killed by Akechi Mitsuhide. Nagahide abandoned the campaign and turned back to help Hashiba Hideyoshi avenge this by killing Mitsuhide. At the meeting in Kiyosu Castle where the future of the Oda clan was discussed, Nagahide supported Hideyoshi's position and gained Echizen Province and Kaga Province to rule over 1,230,000 koku, becoming one of the most powerful retainers and daimyo. However, Nagahide died of illness in 1585 without making any impact at all. There is a conflicting record that Nagahide had not died of an illness, but on seeing Hideyoshi gather more power and eclipsing the Oda clan Nagahide had so long served, he felt that he had not lived up for the good of Nobunaga and the Oda clan as whole and committed suicide.
His son Nagashige later became lord of Shirakawa Castle in northern Japan, and by the time of Nagahide's grandson Mitsushige, the family's 100,000 koku landholding was moved to Nihonmatsu, where they remained for the duration of the Edo Period.