West Sussex County Council has opened ‘Stage 6’ of the new road, which links the B2112 Wivelsfield Road with the A272 Lewes Road.
The 0.6 mile stretch of carriageway connects to the new Crest Nicholson housing development, situated on the site of the former St Francis Hospital.
The road, which includes new roundabouts at Fox Hill and on the A272 Lewes Road, cost £6.8m and was funded by developer Crest Nicholson through a highway agreement with the County Council.
The county council says that wildlife and the environment have been protected by building a special arch to take an existing watercourse under the road, badger tunnels and a balancing pond for surface water drainage.
Other improvements include installing a pre-cast concrete culvert for drainage, acoustic fencing to cut back on noise pollution, and a vehicle restrain barrier for safety.
The new road will eventually become part of the A272 once Stage 5 of the relief road, which links Bolnore Village to Rocky Lane, is completed.
Stage 5 is under construction and is expected to be completed by summer 2015.
Once all the Haywards Heath Relief Road is ready, traffic currently forced to travel through the town centre will be diverted along it.
County Council Cabinet Member for Highways and Transport, Pieter Montyn, said: “Opening this section of new road to the public is like fitting the penultimate piece in a jigsaw.
“We now look forward to Stage 5 being finished at the earliest opportunity.
“About eight million vehicles journey through Haywards Heath town centre each year, so once the new road opens it will act as a release to all the traffic pressure there.”
County Local Committee Chairman for North Mid Sussex, Pete Bradbury, added: “We welcome this progress and look forward to when the whole relief road is open.”