The garden, which overlooks Balmaha Bay, was opened by US national parks ambassador Lee Stetson, along with Scots magazine editor Robert Wight.
It marks the completion of the Â£130,000 transformation of the site.
A statue of Tom Weir was unveiled in December 2014, 100 years after the broadcaster's birth.
The author and environmentalist, who was best known for his long-running television series Weir's Way, died in 2006.
The mountain garden has been designed by landscape specialist and Scottish Campaign for National Parks chairman, Ross Anderson, and was developed by local landscape gardeners.
It features a range of typical mountain plants, as well as a selection of stones and rocks collected near the Highland Boundary Fault which runs through Loch Lomond close to Balmaha.
Slate slabs, engraved with words about Loch Lomond written by Tom Weir in the Scots Magazine 50 years ago, can also be seen in the garden.
Mrs Weir, 96, was present at the opening of the garden.
The site was leased to the Friends of Loch Lomond and The Trossachs by Stirling Council.
Friends chairman James Fraser, said: "This event is an integral part of a visit by Lee Stetson as part of the centenary celebrations of the US National Parks Service, and also another opportunity to remind everybody of how Tom Weir became Scotland's most loved mountain man.
"It is also a fitting way to mark the completion of work at the Tom Weir's Rest site which has become firmly established in a relatively short period of time as one of Loch Lomond's top visitor attractions with over 100,000 visitors so far."
A mountain garden in honour of countryside broadcaster Tom Weir and his widow Rhona has been formally opened on Loch Lomondside.