The 33-year-old British royal got engaged to the 'Suits' actress earlier this month after 16 months of dating and the prince believes his beloved parent - who died in a car accident in Paris, France, in 1997 - would have been as ''thick as thieves''.
Asked what his mother would have made of his bride-to-be, Harry replied: ''Oh they'd be thick as thieves, without question.''
The former soldier admitted he felt his mother's absence especially keenly during the landmark moments in his life, like the announcement of his engagement to Meghan, 36.
But Harry explained that because the brunette beauty's engagement ring features two stones from his mother's collection, he can still feel Diana's presence.
Speaking to the BBC, Harry explained: ''I think she would be over the moon, jumping up and down, you know so excited for me, but then, as I said, would have probably been best friends with Meghan.
''It is days like today when I really miss having her around and miss being able to share the happy news.
''But you know with the ring and with everything else that's going on I'm sure she ...''
But before the royal could complete his sentence, Meghan said: ''She's with us.''
Harry - who is poised to marry the former 'Suits' star in the spring of next year - also believes his mother would have struggled to have contained her excitement at the announcement.
He said: ''I'm sure she's with us, jumping up and down somewhere else.''
News of the couple's engagement was announced by Prince Charles, Harry's father, on Monday (27.11.17).
Following the announcement - which had been rumoured for months - the high-profile duo appeared at a photo call at Kensington Palace, in the White Garden, which is dedicated to Diana.
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