The interior ministry said the 12 had been captured in different areas across Kuwait. They had been on the run since their sentencing last month, while two other convicted Kuwaitis remained at large.
The supreme court in Sunni-ruled Kuwait, which has a sizeable Shiite minority, in June overturned an acquittal by an appeals court and convicted 21 Shiites of forming a "terrorist cell" with ties to Iran and Hezbollah.
The cell had planned to launch attacks across the Gulf state, according to the court verdict.
Last month, authorities expelled 15 Iranian diplomats and shut down the military, cultural and trade missions of the Iranian embassy over Tehran's backing of the "terrorist cell".
Iran said the allegation is baseless.
Shiites account for around a third of Kuwait's native population of 1.35 million.
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