Sökkmímir (Old Norse: Søkkmímir, probably "Mimir of the depth", lit.'Sunk-Mimir') was a jötunn who appears in two sources from Norse mythology, suggesting that he was once a well-known jötunn in Scandinavia.
In Grímnismál, stanza 50, it appears that Odin killed the jötunn:
Sviðurr ok Sviðrir
er ek hét at Søkkmímis,
ok dulða ek þann inn alda iötun,
þá er ek Miðviðnis vark
ins mæra burar
orðinn einbani.
So. I deceived the giant
Sokkmimir old
As Svithur and Svithrir of yore;
Of Mithvitnir's son
the slayer I was
When the famed one found his doom.
Svidur and Svidrir
I was at Sökkmimir's called,
and beguiled that ancient Jötun,
when of Midvitnir's
renowned son
I was the sole destroyer.
He notably appears in Ynglingatal, where subterranean abodes of jötnar are called Sökkmímir's halls: