Starting at Madrid as a baby-faced breakthrough goalkeeping star at the turn of this century, Casillas confirmed the news on Tuesday.
During 16 decorated seasons at Los Blancos, Casillas won five La Liga titles, two Copas del Rey, four Supercopas de Espana, three UEFA Champions League, two UEFA Super Cups and three FIFA Club World Cups.
A career swansong followed at Porto, where he plied his trade for a further four seasons from 2014, winning the Primeira Liga in 2017-2018.
Casillas is arguably best remembered for serving as the impeccable last line of defence as Spain claimed glory at UEFA Euro 2008, the 2010 FIFA World Cup and UEFA Euro 2012.
Those were the fruits of a 1048-game professional career. Here are the remarkable numbers amassed by one of football's modern greats.
REIGNING IN SPAIN
725 – Casillas is second on Real Madrid's all-time appearance roster, behind former team-mate Raul (741).
510 – His 510 LaLiga outings are the seventh most in the history of the competition.
334 – Only Lionel Messi (361) has won more matches in Spain's top flight.
177 – Casillas has the third most clean sheets in LaLiga, behind Andoni Zubizarreta (235) and Paco Buyo (213). Zubizarreta and Buyo also lead the way in terms of games started by goalkeepers (622 and 542 to Casillas' 508) and minutes played (55,746 and 48,717 to 45,684).
KING OF EUROPE
177 – Casillas's number of UEFA Champions League appearances – coincidentally the same amount as his LaLiga clean sheets record – are the most in the competition.
101 – Cristiano Ronaldo is eight appearances behind Casillas on the all-time list and, with 106 wins, he is the only player to have tasted victory more often.
1 – His solitary substitute appearance in the UEFA Champions League came during the 2002 final, as a replacement for the injured Cesar Sanchez to help Madrid close out a 2-1 win over Bayer Leverkusen at Hampden Park.
57 – Casillas's number of clean sheets lead the way in Europe's top competition, with Gianluigi Buffon (51) and Edwin van der Sar (50) the only others to have chalked up half a century.
3 – He is alongside Juan Alonso and Keylor Navas as a three-time winner while playing in goal for Real Madrid.
167 – Casillas's cap record for Spain was recently passed by Sergio Ramos (170). Both share a joint-record of 17 appearances in FIFA World Cup finals.
0 – He did not concede a single goal in the final three matches of the triumphant UEFA Euro 2008, 2010 FIFA World Cup and UEFA Euro 2012 campaigns, completing a hat-trick of quarter-final, semi-final and final clean sheets.
2 – The two goals he allowed in South Africa are the joint fewest let in by a FIFA World Cup-winning goalkeeper alongside Buffon in 2006 and Fabian Barthez in 1998. His five clean sheets matched the efforts of Buffon, Barthez and Brazil's triumphant 1994 goalkeeper Taffarel.
433 – Casillas's run of minutes without conceding a goal at the 2010 FIFA World Cup is the best for a Spain goalkeeper.
2 – Republic of Ireland's Ian Harte in 2002 and Paraguay's Oscar Cardozo in 2010 had penalties saved by Casillas. Excluding shootouts, only Poland's Jan Tomaszewski and the United States' Brad Friedel have saved two penalties at FIFA World Cups.