Former Liverpool midfielder Dandha, 23, joins on a two-year contract from Swansea.
County have also signed Canadian midfielder Victor Loturi on a three-year deal.
"Yan is a player that our recruitment department have targeted for a period of time," said County manager Malky Mackay.
"Yan's talent will certainly complement our group and we also feel he is a great fit for what we are trying to build here and the culture we have created.
"Victor is a really exciting young talent that we are delighted to bring to Ross County.
"At 21 he has really good and valuable experience in the men's game and is a player we feel we can help develop."