The Welsh Labour general secretary, Chris Roberts, is to leave his post in the autumn after running the party for five years.
Mr Roberts said he was leaving the role to pursue new challenges.
He will go six months before the Welsh assembly election in May 2011.
Welsh Labour leader and First Minister Carwyn Jones said: "Chris has been an excellent stabilising presence inside Welsh Labour during what could have been a turbulent period for the party."
Mr Roberts said he felt that five years was "about right" for the job.
In a message sent to party members, Mr Roberts said: "I feel this is the appropriate time for me to embark on new challenges.
"Given the crowded Welsh electoral timetable there is never a convenient time for a change in personnel but I believe now is the time that will cause least disruption to the party in Wales."
Mr Jones added: "I have enjoyed my time working with Chris, he always maintains a sense of humour even when the going is tough and I know he will be missed by politicians and members alike.
"I wish him the very best for the future."