"17 aircraft (of the 21) crossed the median line and its extension, entered our northern, central, and southwestern (air defence identification zone), and joined PLA vessels for joint combat patrol," it said in a statement posted on X around 11:30 am.
Taiwan's armed forces "are monitoring the activities with our joint surveillance systems, and have dispatched appropriate assets to respond accordingly."
The median line bisects the Taiwan Strait, a narrow 180-kilometre (110-mile) waterway separating the island from mainland China.
Beijing does not recognise the line as it claims democratic Taiwan as part of its territory. It has also never renounced the use of force to bring the island under its control.
China sends warplanes and naval vessels around Taiwan on a near daily basis -- a move experts say is a form of "grey-zone harassment", stopping short of an outright act of war but enough to exhaust Taipei's armed forces.
According to the defence ministry, the 21 aerial objects detected Saturday included J-16 fighter jets and Y-8 medium-range transport aircraft, as well as drones.
The highest number around Taiwan so far this year was in March, when the ministry said 36 Chinese aircraft were detected in a single 24-hour period.
Last year's record was in September when Beijing's military sent 103 aircraft -- 40 of which crossed the median line -- in a 24-hour period.
Saturday's show of force comes a day after China activated two aviation routes that run close to Taiwan's outlying islands of Kinmen and Matsu.
Taipei's Civil Aviation Administration expressed "solemn protest against China's unilateral measures without consultation" on Friday.
The new routes make the airspace separation between the two sides "very narrow", it said, increasing flight safety risks during bad weather or abnormal flight operations.
China's aviation authority also said Friday the airspace around Fuzhou Changle Airport -- 30 kilometres from the closest outlying Taiwanese island -- would be "further optimised and adjusted" on May 16, four days before the inauguration.
Under the administration of Taiwanese President Tsai Ing-wen, tensions between Beijing and Taipei have ramped up, as she and her government do not acknowledge China's claim.
Her deputy, Vice President Lai, won elections in January despite warnings from Beijing that he would be the cause of "war and decline" for Taiwan.
China regards Lai -- who used to be outspoken about Taiwan independence -- as a "dangerous separatist", though he has moderated his views in recent years.
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