The Ominous Decade is a traditional term for the last ten years of the reign of King Ferdinand VII of Spain, dating from the abolition of the Spanish Constitution of 1812, on 1 October 1823, to his death on 29 September 1833.

The decade saw an endless series of riots and attempts of revolutions, such as that of Torrijos, funded by English liberals, on 11 December 1831. Aside from the liberal side, Ferdinand's policies caused discontent also in the conservative party: in 1827 a revolt broke out in Catalonia, and later extended to Valencia, Aragon, the Basque Country and Andalusia, spurred by ultra-reactionaries according to whom Ferdinand's restoration had been too timid, failing in particular to reinstate the Inquisition. In what was called the War of the Agraviados, some 30,000 men controlled most of Catalonia and some of the northern regions, and even established an autonomous government. Ferdinand intervened personally, moving to Tarragona to quench the revolt: he promised an amnesty, but once the rioters had surrendered, he had their leaders executed and others exiled to France.

Further instability came when, on 31 March 1830, Ferdinand issued the Pragmatic Sanction, which had been approved by his father Charles IV as early as 1789, but not published till then. The decree allowed the succession to the Spanish throne also to female heirs, in case a male one was not available. Ferdinand would have only two children, both daughters, the eldest being the future queen Isabella II, who was born in October 1830. The Sanction excluded from the succession Ferdinand's brother, Carlos, Count of Molina.