"It's great to be involved with such a vivid following as Ulster," said Botha.
"The move is a great opportunity to see Ireland and the United Kingdom."
Botha, who has 11 international caps to his name, could still challenge for a place in the Springbok squad for the 2009 Lions tour but head coach Peter de Villiers has publicly stated that he would prefer his players to be based at home in the lead up to next summer's series.
"I'm really looking forward to playing in Europe," said the 28-year-old tight-head. "I feel that the move to Ulster will be a great opportunity for both me and my family.
"I want to experience the famous Northern Ireland culture first hand and I'm sure we'll end up venturing across to mainland Europe for holidays, which we are all excited about.
"Meeting new people and seeing new places is always exciting and I can't wait to join the rest of the squad and get stuck into some serious training for the new season."
Former Scotland supremo and now Ulster coach Williams was full of praise for his new South African acquisition, who becomes his sixth overseas signing of the pre-season.
"BJ is a world-class player and is one of the best tight-head props in the game," said Williams.
"He was part of the South African squad that won the World Cup and is used to being part of a winning team.
"BJ will bring with him the culture of excellence and winning, a mindset I hope he can pass onto our younger players."