The Duchess of Sussex shared a heartfelt letter and a gift of some "delicious" snacks with the activists.
Shannon Watts, the founder of Moms Demand Action, revealed that her non-profit organisation had received the will-wishes from Meghan on Twitter.
In the tweet, she described how "moving" it was to receive a gift from the former TV star.
She wrote: "Staff have been working around the clock for weeks, so it was moving to receive such a supportive note – and snacks! – from the Duchess of Sussex today."
Meghan Markle
DYLAN MARTINEZ
Meghan with Prince Harry at the Invictus Games earlier this year
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Ms Watts later added that the snacks consisted of: "Doughnuts, bagels w/the works, coffee, cookies, etc… Delicious and appreciated.”
In her typed note, Meghan detailed how grateful she was for the group's work and the "dedication" behind it.
She said: “Good afternoon ladies!
“To each of you and your fearless leader, Shannon, a huge thank you for the work you are doing to keep our kids, families, and communities safe.
“Please know that while you may feel exhausted by your endless hours of dedication to this cause, I hope you feel energised in knowing that the work you’re doing is vital and so deeply appreciated."
Members of the Royal Family are not normally expected to intervene or express opinions on matters of politics.
But both Meghan and husband Prince Harry have repeatedly shared their personal view since their decision to step down from the Firm in January 2020.
On its official website, Moms Demand Action is described as a “grassroots movement of Americans fighting for public safety measures that can protect people from gun violence”.
The focus of the movement includes passing “stronger gun laws” and working “to close the loopholes that jeopardise the safety” of friends and family.
Earlier this month, photos of Meghan working with Dutch nonprofit Project Fearless were also released.
In a statement, she said: “Safe, supportive spaces like Project Fearless allow girls the opportunity to express themselves, build each other up, gain resilience, and tackle new challenges.”