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You can reference those remote tracking branches ~(listed with git branch -r ) with the name of their remote.

You need to fetch the remote branch:

git fetch origin aRemoteBranch
If you want to merge one of those remote branches on your local branch:
git checkout aLocalBranch
git merge origin/aRemoteBranch
Note 1: For a large repo with a long history, you will want to add the --depth=1 option when you use git fetch.
Note 2: These commands also work with other remote repos so you can setup an origin and an upstream if you are working on a fork.
Note 3: user3265569 suggests the following alias in the comments:
From aLocalBranch, run git combine remoteBranch

Alias:
combine = !git fetch origin ${1} && git merge origin/${1}
Opposite scenario: If you want to merge one of your local branch on a remote branch (as opposed to a remote branch to a local one, as shown above), you need to create a new local branch on top of said remote branch first:
git checkout -b myBranch origin/aBranch
git merge anotherLocalBranch
The idea here, is to merge "one of your local branch" (here anotherLocalBranch) to a remote branch (origin/aBranch).

For that, you create first "myBranch" as representing that remote branch: that is the git checkout -b myBranch origin/aBranch part.

And then you can merge anotherLocalBranch to it (to myBranch).
                @knocte No: "If you want to merge one of your local branch on one of those remote branch": I am merging "aLocalBranch" to "myBranch", with "myBranch" representing a remote branch origin/aBranch.
– VonC
                Jan 19, 2016 at 5:37
                Sorry to bother you @VonC, in your previous comment you say aLocalBranch is not a typo, but you approved the latest edit (2 years later!) which corrects this "probable typo". I wanted to check with you before undoing the edit.
– rath
                May 1, 2019 at 13:36
                @rath You are correct: it appears I might have reviewed that edit a bit hastily, bordering on carelessness. I did edit the answer to clarify the second merge case: can you tell me if this is clearer now?
– VonC
                May 1, 2019 at 18:54
                I've created an alias for this. From aLocalBranch  run git combine remoteBranch. Alias: combine = !git fetch origin ${1} && git merge origin/${1}
– user3265569
                Feb 9, 2021 at 15:42
Whenever I do a merge, I get into the branch I want to merge into (e.g. "git checkout branch-i-am-working-in") and then do the following:
git merge origin/branch-i-want-to-merge-from
                This worked for me when the accepted answer reported Not something we can merge. In my case I was merging from another user’s fork of my repo on GitHub.
– SJT
                Jul 5, 2019 at 12:59
                Does the first command fetch required? Does ‘git merge origin/remote_branch_name’ reads from remote branch, not local. So I shouldn’t care, is local branch updated or not?
– Michael Freidgeim
                Apr 30, 2020 at 6:45
                @MichaelFreidgeim You do need to fetch first so your local repository knows the state of remote. Git pull is basically a fetch and merge from the current remote version of your current branch, but now you fetch and merge from a different one. stackoverflow.com/questions/292357/…
– DZet
                Jul 6, 2020 at 17:18
Set this newly created branch to track the remote branch:
  git branch --set-upstream-to=origin/remote-branch new-local-branch
Enter into this branch:
  git checkout new-local-branch
Pull all the contents of the remote branch into the local branch:
  git pull
If you already fetched your remote branch and do git branch -a,

you obtain something like :  
* 8.0
  remotes/origin/xxx
  remotes/origin/8.0
  remotes/origin/HEAD -> origin/8.0
  remotes/rep_mirror/8.0
After that, you can use rep_mirror/8.0 to designate locally your remote branch.
The trick is that remotes/rep_mirror/8.0 doesn't work but rep_mirror/8.0 does.
So, a command like git merge -m "my msg" rep_mirror/8.0 do the merge.
(note : this is a comment to @VonC answer. I put it as another answer because code blocks don't fit into the comment format)
        
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