Her Majesty, 95, had cancelled a planned trip to Northern Ireland last week after doctors advised her to rest.
It is the first time the monarch has been seen since she hosted an evening reception for the global investment summit last Tuesday, with guests including Microsoft co-founder Bill Gates and US climate enjoy John Kerry.
She was pictured on a computer screen as she greeted the ambassador from the Republic of Korea, Gunn Kim, who was dressed in traditional Korean clothing. She was accompanied by Mrs Hee Jung Lee.
The Queen was wearing a yellow dress with button detail and her three-strand pearl necklace and was seen smiling during the audiences on Tuesday.
It comes just days after it emerged that the monarch had spent the night at King Edward VII hospital in Marylebone - her first overnight stay at a medical facility in eight years. Aides stressed her stay was due to practical reasons.
The monarch is being looked after by the Medical Household, her expert team of royal physicians.
The Queen is due to attend events in Scotland next week as part of her COP26 engagements. She will be joined by the Prince of Wales, the Duchess of Cornwall and the Duke and Duchess of Cambridge between November 1-5.
It has not yet been confirmed as to whether she will travel to Scotland.
The engagements on Tuesday follow claims that the Queen is “knackered” after a packed schedule of royal engagements. One courtier told the Sunday Times it had all been “too much”.
The Queen was said to be in “good spirits” after her hospital stay and back at her desk, undertaking light duties. However, she missed a church event on Wednesday.
Buckingham Palace would not comment on whether the monarch has received her booster Covid jab.