These two have replaced South African legend Jacques Kallis and Simon Katich respectively.
Bayliss, the head coach of England, guided them to the World Cup title in 2019.
Here is more.
IPL: Bayliss is back!
Great news for KKR fans. Trevor Bayliss and @Bazmccullum are back , as Head Coach and Batting coach/Mentor respectively. You couldn't have got a better combo to turn things around! pic.twitter.com/4UFttASUOI
Bayliss to step down as England's coach in September 2019
A few days back, Bayliss had confirmed that he no longer intends to continue with England once his tenure ends in September 2019.
"I have always been a believer that four or five years is long enough, whether you are doing well or not. It is time for a new voice for the boys, to hopefully take them to another level," said Bayliss.
Bayliss helped England win 2019 World Cup
Under his guidance, England managed to win the 2019 World Cup last Sunday.
He had taken over as England's head coach in May 2015.
Bayliss helped England reach the number one ranking in ODIs and saw them dominate the circuit in the last couple of years.
England also won the 2015 Ashes at home (3-2) under Bayliss.
It was under his guidance that the eastern giants won two IPL trophies in 2012 and 2014.
KKR seized the opportunity to get Bayliss back after Kallis decided to call it quits.
The team will want to see Bayliss weave his magic once again.
The looks like a good decision by KKR.
McCullum to help KKR as batting mentor
Meanwhile, Brendon McCullum started his IPL career with KKR in 2008.
He spent five seasons with the franchise.
McCullum, who had a successful stint in the IPL across franchises, was seen commentating in the World Cup.
He will seen in action for the Toronto Nationals in the second season of the Global T20 Canada. It will be held between July 25 to August 11.
KKR hope to shine under Bayliss and Baz
KKR will be hoping to do well next season after a disappointing IPL 12 campaign. With the arrival of Bayliss and Baz, they have two solid customers to help out the side. It will be an exciting phase for Kolkata.