Salisbury Cathedral Spire will be lit in green on Saturday night to celebrate the award, which has been given by the Christian environmental group A Rocha UK.
To award the status of an eco diocese, a diocese or parish's green credentials are looked at in five areas: worship, teaching, buildings, land, community engagement and lifestyle.
Andy Atkins, CEO of A Rocha UK said: "When we launched Eco Church in 2016 we had high hopes and expectations. To have reached this far so quickly is humbling, exciting and such a huge encouragement that so many churches in a diocese are working together to care for God's earth should be an encouragement to us all".
The Eco Diocese award is part of the wider Eco Church scheme. To achieve an Eco Diocese Bronze Award, 10% of Salisbury diocese's 500 local worshipping communities, which are spread across Dorset and Wiltshire, engaged with Eco Church and 5% obtained an Eco Church Award.
In addition, the whole diocese met Eco Church Bronze Award standards for its management of buildings and land. There was also a requirement for ethical investment of diocesan funds and for the inclusion of care for creation in training for ministry.
The Bishop of Salisbury, Rt Revd Nicholas Holtam said: "I am delighted and very encouraged that the Diocese of Salisbury is the first to win this award. As the lead bishop on the environment for the Church of England I am delighted my own diocese is first and I also hope that others will be close behind us,
"The recent IPCC report on climate change and the WWF report on the destruction of biodiversity means the care of God's earth is urgent. I hope that becoming an Eco Diocese will encourage us to do more and better. "
"Our target to become an Eco Diocese was to have 50 churches registered and now we have 70 and we have Bronze, Silver and Gold awards. And we have more churches still working towards getting their own award.
"It shows that as a Diocese we recognise that the care for God's creation is at the heart of ministry and mission".
David Morgan, Chair of the Diocesan Environmental Group said: "It is exciting and encouraging to see caring for creation come up the agenda of many parishes in the diocese. Some of our churches have been on this journey for many years; for others, it is a new understanding of our responsibilities which they have embraced and acted upon.
"This proves that even with a listed church building, it is very possible to achieve an award."
Announcing the award, Colin Brady, Social Justice Programme Manager for the Diocese of Salisbury said: "Last week we received news of two more churches gaining their Bronze Eco Church award, and notification that our offices in Salisbury had met the criteria for its own Bronze award, and suddenly we were there. And we are confident that more local awards will follow."
"Many of our congregations have had lots of fun building bug hotels in their churchyards, adding a few more points towards their Eco Church awards with a clear message that Christians care for the environment."
The quest for Eco Diocese status was helped by some good news early in the process, with Hilfield Friary near Dorchester gaining the first Gold award in the country.
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