The Queen has signed a charter setting out the Commonwealth's values and commitment to equal rights.
She made her first official public appearance in more than a week at the Marlborough House reception in London.
Earlier, the Queen was forced to pull out of the annual Commonwealth Day service at Westminster Abbey, for only the second time since becoming monarch.
Buckingham Palace said she was still recovering from the symptoms of gastroenteritis.
The Commonwealth, which accounts for 30% of the world's population, has for the first time drawn up a charter that details 16 core beliefs, which was adopted by all 54 member states in December.
These include upholding democracy and opposing "all forms of discrimination" although it does not explicitly include discrimination against gay people. In some Commonwealth countries, homosexual acts are illegal.
The charter states: "We are implacably opposed to all forms of discrimination, whether rooted in gender, race, colour, creed, political belief or other grounds."
Guests at Marlborough House including Commonwealth high commissioners heard the Queen say the charter "represents a significant milestone as the Commonwealth continues its journey of development and renewal".
She told them: "We have now, for the first time, a single document that captures the core values and aspirations of the Commonwealth and all its members."
Last week the Queen, 86, was admitted to hospital for the first time in 10 years, suffering from the symptoms of gastroenteritis.
The monarch saw a doctor on Monday morning and the decision was taken that it would be best if she did not sit through an hour-long church service, said BBC royal correspondent Peter Hunt.
Officials insist the Queen's health problems are "nothing serious " and "nothing to fret about".
A palace spokeswoman said: "This time last week she was in hospital, but she's in great spirits and apart from this is in good health and will be going to the reception in the evening.
"It's just the tail end of the symptoms, her condition has not worsened at all."
The Queen hopes to undertake some of her official engagements planned for the rest of this week, the palace said.
The Duke of Edinburgh attended the service as planned which was also attended by Commonwealth ambassadors and featured an address from tycoon Richard Branson.
The only other time the Queen withdrew from the same event was in 1993 when she was suffering from flu.
In her annual pre-recorded message, which formed part of the service, the Queen spoke about the benefits of the Commonwealth.
She said: "Our shared values of peace, democracy, development, justice and human rights - which are found in our new Commonwealth Charter - mean that we can place special emphasis on including everyone in this goal, especially those who are vulnerable."
Our correspondent said that to its critics the Commonwealth was ineffective, but its supporters remember the moral authority it brought to the campaign against apartheid.
For the Queen, it has been an important dimension of her reign.
All official engagements last week, including a visit to Rome, were either cancelled or postponed after the Queen was treated at London's King Edward VII's Hospital.