The former Ireland international flanker captained the Scarlets for five consecutive seasons, and concluded a distinguished career having played more than 50 European games, 201 games (19 tries) for Llanelli and the Scarlets with 65 caps for Ireland plus the honour of two British and Irish Lions caps.
The appointment comes after the departure of former Scarlets Head Coach Nigel Davies who has left the region to take up a position as the new Director of Rugby at Gloucester.
Easterby will lead a strengthened Scarlets coaching group alongside Backs Coach Mark Jones, Strength and Conditioning Coach Brad Harrington and the newly appointed Forwards Coach Danny Wilson. The appointment ensures a strong structure and continuity and consistency of environment, values and approach for the playing squad within a close-knit Scarlets set-up. The Scarlets will also work with the WRU on the appointment of another specialist skills coach.
Easterby said: "I am delighted to have been given the opportunity to take rugby forward as Scarlets Head Coach, it is a real honour and privilege for me. I've had lots of individual challenges in my life and this will be one of the biggest of my career, but I have always pushed myself hard and set myself high standards. I will give it my all and I'm grateful to the Chairman and board of directors who are putting their faith in me.
"The values instilled deep within the Scarlets have been in place for almost 140 years, so like Nigel and Gareth before me I will ensure we never lose sight of who we are as well as putting my own stamp on our ambitions and our game.
"The Scarlets has been the biggest part of my professional career, its continued success is what drives me and motivates me. Being given the opportunity to coach and now become Head Coach proves the succession plans are working and the club and board have the confidence in me to take things forward.
"Hopefully with the history I have here with our club, we move forward with real excitement while keeping that Scarlets culture right in the heart of what we do."
Scarlets Chief Executive Mark Davies said: "Under Nigel Davies' guidance, the development of our coaching group over the last two years has been as significant as that of our players, with Simon (Easterby) and Mark (Jones) taking ownership of key areas of performance on the field.
"The Scarlets philosophy is to develop our rugby future from within our region and provide opportunities for talented individuals, and this applies as much to our coaches as it does our players."