Calling it a "clear violation" of 2018 agreement, South Korea said none of the shells crossed the Northern Limit Line, the de facto maritime border between the two countries.
North Korea has fired a barrage of artillery shells into a maritime buffer zone, the latest in a series of launches by an increasingly belligerent Pyongyang.
About 130 artillery rounds were simultaneously fired at 0559 GMT (14:59 local) from two separate sites on Monday, one on North Korea's east coast and one on the west coasts, Seoul's military said.
The South's Joint Chiefs of Staff said the barrage was a "clear violation" of the 2018 agreement between the North and South that established the buffer zone in a bid to reduce tensions.
It said none of the shells crossed the Northern Limit Line, the de facto maritime border between the two countries.
The military said it had issued "several warnings" over the barrage, without giving any further details.
"Our military is strengthening its readiness posture in preparation for emergencies while tracking and monitoring related developments under close cooperation between South Korea and the United States," it added.
Record-breaking missile launches
At a summit in Pyongyang in 2018, former South Korean president Moon Jae-in and the North's Kim Jong Un agreed to establish buffer zones along land and sea boundaries in a bid to reduce tensions.
But since talks collapsed in 2019, Kim has doubled down on his banned weapons programmes, and experts say he may now be testing South Korea by violating the buffer zone agreement.
Pyongyang has fired artillery into the buffer zone repeatedly in recent months.
It has also conducted a record-breaking blitz of missile launches in recent weeks, including its newest intercontinental ballistic missile (ICBM) last month, the most powerful such test by the nuclear-armed country yet.
Pyongyang, which is banned from testing ballistic missiles by repeated UN Security Council resolutions, has repeatedly claimed its weapons tests are a legitimate response to Washington's moves to boost the protection it offers to allies Seoul and Tokyo.
Officials and analysts in Seoul and Washington say the launches may build up to a seventh nuclear test.