The 27-year-old midfielder signed on Friday and will add depth to Sam Allardyce's squad as the Upton Park outfit look to stave off the threat of relegation from the Premier League.
Armero can play at left-back or on the left wing and his versatility may come in handy for Allardyce.
The Colombia international began his career in his home country and stints at Palmeiras, Udinese and Napoli have followed.
He initially joined the Naples-based club on loan in January last year, but signed a permanent deal in the close-season after impressing.
Rafael Benitez's Napoli also completed the signing of Algeria defender Faouzi Ghoulam from Saint-Etienne on Friday.
The 22-year-old left-back - who is likely to fill Armero's role in the squad - has signed a four-and-a-half year contract at Stadio San Paolo.
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