What is the longest surname in the world?

It's certainly not mine, which runs to just three letters!

The longest recorded name is that of one Hubert Blaine Wolfeschlegelsteinhausenbergerdorff. This German from Hamburg migrated to Philadelphia, USA, and has had his name checked out by the Guinness World Records organisation.

The longest personal name is 747 characters long, and belongs to Hubert Blaine Wolfe­schlegel­stein­hausen­berger­dorff Sr. (b. 4 August 1914, Germany) who passed away on 24 October 1997, in Philadelphia, Pennsylvania USA, as verified on 1 January 2021.

https://www.guinnessworldrecords.com/world-records/67285-longest-personal-name

Hubert Blaine Wolfeschlegelsteinhausenbergerdorff's complete name is:

Adolph Blaine Charles David Earl Frederick Gerald Hubert Irvin John Kenneth Lloyd Martin Nero Oliver Paul Quincy Randolph Sherman Thomas Uncas Victor William Xerxes Yancy Zeus Wolfeschlegelsteinhausenbergerdorffwelchevoralternwarengewissenhaftschaferswessenschafewarenwohlgepflegeundsorgfaltigkeitbeschutzenvonangreifendurchihrraubgierigfeindewelchevoralternzwolftausendjahresvorandieerscheinenvanderersteerdemenschderraumschiffgebrauchlichtalsseinursprungvonkraftgestartseinlangefahrthinzwischensternartigraumaufdersuchenachdiesternwelchegehabtbewohnbarplanetenkreisedrehensichundwohinderneurassevonverstandigmenschlichkeitkonntefortpflanzenundsicherfreuenanlebenslanglichfreudeundruhemitnichteinfurchtvorangreifenvonandererintelligentgeschopfsvonhinzwischensternartigraum.

https://en.wikipedia.org/wiki/Hubert_Blaine_Wolfeschlegelsteinhausenbergerdorff_Sr.