Overlooking The Mall, the venue can accommodate between 50-100 guests for theatre-style presentations and informal receptions. The Royal Society has said the space will also be able to host a smaller number for formal dining.
Books and archives from the library itself have been moved within the building and are still available for visitors to view.
The refurbishment also covered a revamp of the venue's kitchen space, boosting its dinner cover capacity to 1,000 guests. The library joins the 12 existing rooms within the venue that can be hired for private use.
Nicholas da Costa, head of conference services at the Royal Society, said: "The Old Library is open for business. The Royal Society has hosted some of the most important events in the scientific world, so we are delighted to make our venue as widely accessible as possible. The new room lets us keep pace with demand and contains brand new IT and AV equipment."
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