Grounds, who penned a new two-year deal with Boro in June, will remain at Easter Road until January 29.
The 22-year-old made 20 appearances for Gordon Strachan's side last season, but has been restricted to Carling Cup duty so far this season.
Grounds joins Dickoh in strengthening the Hibs defence after the Ghana international's arrival from Utrecht was confirmed.
The Danish-born 27-year-old, who has been capped 13 times, has signed a one-year deal.
Former Nordsjaelland defender Dickoh told the club's website: "I am really pleased to have signed for Hibernian - a big Scottish club with a great stadium and excellent training facilities.
"As soon as I was taken around the training centre, I knew that I wanted to be part of Hibernian and I think Edinburgh is a beautiful city; a perfect place to live.
"I would like to be able to make an impact straight away in the Hibernian team and help the club to challenge for honours.
"I am an ambitious player and want to fulfil my career goals here at Hibernian.
"The stadium is a great setting for a successful team and I'm very excited about what we can achieve at the club."
Manager John Hughes said: "It is great to have signed Francis - he is a quality player who has proven himself in both Denmark and Holland.
"He will bring a good physical presence and experience to our defence. I'm looking for him to be a strong character in the dressing room.
"Francis has been about and played at international level - he can become an important player for Hibernian."