Security teams at Sheremetyevo Airport detained the woman for interrogation as bomb disposal officers were summoned.
Part of the busy airport was sealed off.
The lookalike weapon showed on a scanner during a compulsory luggage check at a terminal entrance .
The London woman - whose picture has not been released. - was searched and faced questioning by FSB security service officers before being released and allowed to fly home.
The fake weapon was impounded by Russian airport security officers.
It is believed to be not a toy but a lookalike grenade used by the Russian military for training exercises.
Pictures showed the item after her bags were opened during a search at Sheremetyevo airport.
The teacher said that she had bought “grenade” at a street market in Moscow and intended too show it to her students during a History lesson at her London school.
She was on a short pre-Christmas holiday to the Russian capital.
The incident was on 21 December - the same day foreign secretary Boris Johnson flew to Moscow.
However, details were only released today.
Russia has a high level of security at airports ahead of a year that will see both presidential elections and the finals of the FIFA World Cup.
The capital's airports have been previously attacked by terrorists.
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