How to know from which yum repository a package has been installed?

With yum-utils installed, repoquery will provide the information you seek (here 'epel' being the repository).

$ repoquery -i cherokee

Name        : cherokee
Version     : 0.99.49
Release     : 1.el5
Architecture: i386
Size        : 8495964
Packager    : Fedora Project
Group       : Applications/Internet
URL         : http://www.cherokee-project.com/
Repository  : epel
Summary     : Flexible and Fast Webserver
Description :
Cherokee is a very fast, flexible and easy to configure Web Server. It supports
the widespread technologies nowadays: FastCGI, SCGI, PHP, CGI, TLS and SSL
encrypted connections, Virtual hosts, Authentication, on the fly encoding,
Apache compatible log files, and much more.

What version of yum?

On the current version if the installed package is the same version as the most recent one available then the repo it was installed from is shown.

$ yum info irssi
Installed Packages
Name       : irssi
Arch       : i586
Version    : 0.8.14
Release    : 1.fc11
Size       : 2.3 M
Repo       : installed
From repo  : updates
Summary    : Modular text mode IRC client with Perl scripting
URL        : http://irssi.org/
License    : GPLv2+
Description: Irssi is a modular IRC client with Perl scripting. Only text-mode
           : frontend is currently supported. The GTK/GNOME frontend is no
           : longer being maintained.

$ yum --version
3.2.23

If there is a newer package available, then it will be shown separately, with the new version showing the repo it's available from.

Coming way to late but (at least on Fedora 15) one can use yumdb for similar queries:

yumdb info 'python*'

And what I actually needed to list packages from given repo(s):

yumdb search from_repo 'my-*-repo'

If all else fails, you can inspect the yumdb manually. It's located in /var/lib/yum/yumdb and contains detailed information on every installed package. You'll be particularly interested in from_repo. For example, for the bind-utils package:

# for i in /var/lib/yum/yumdb/b/*bind-utils*/*; do echo $i: `cat $i`; done
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/checksum_data: 39f7840f93d3d76825a9e8da79cfe0366f7fad68f018a273477aee62cccfa3f4
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/checksum_type: sha256
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/command_line: install bind-utils
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/from_repo: updates
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/from_repo_revision: 1397654451
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/from_repo_timestamp: 1397654759
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/installed_by: 0
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/reason: user
/var/lib/yum/yumdb/b/73ea08770fa666e18c59842bf65fa0f3a0b103d8-bind-utils-9.8.2-0.23.rc1.el6_5.1-x86_64/releasever: 6

To get just the information you want:

# cat /var/lib/yum/yumdb/b/*bind-utils*/from_repo
updates

Later versions will provide even more data; for instance in EL 7 the exact mirror and URL from which the package was downloaded is stored here.

rpm -qi packagename

Will tell you the vendor and packager

To see what installed from repo epel: (Should work under Centos 5.5) note, if you have no repo named epel, it will spit out everything installed.

repoquery --repoid=epel -a | xargs yum list installed

The simplest and most straightforward approach without installing any extra tools, greping or outputting extraneous information is simply:

yum list installed [package name]

The third column will provide the ID of the repository the package was installed from.

This was mentioned in a comment on the original question using grep, but that's not necessary. You can just pass the package name directly to yum to filter.

If the package was installed recently, you can look in /var/cache/yum.

Within that directory, there is a directory for each repo, and in that a packages directory. So, you would do something like:

find /var/cache/yum -name [package-name]*

However, cache has to be enabled in your /etc/yum.conf file:

cachedir=/var/cache/yum
keepcache=1

Note that a yum clean [packages|all] will clear out the cache directory.

If the cache directory is empty, there is an alternative way. The information that is read by yum info [package] comes from /var/cache/yum/[repo]/primary.xml.gz

You can look through the file by entering:

gunzip -d -c /var/cache/yum/[repo]/primary.xml.gz | grep '<name>[package]'

However, on machines where yum info [package] does not display "From repo : ", as indicated by 'theotherreceive', it is because it is not in the file primary.xml, so there will be no way to retrieve that information. Therefore, if the package is in two or more primary.xml files, you will have to determine the repo priority on you system.

Based on and answer by Swoogan
On RedHat and CentOS one can do

sudo grep -ir PACKAGE_NAME /var/cache/yum/

The results should look something like

Binary file /var/cache/yum/REPOSITORY_NAME/primary.xml.gz.sqlite matches

Where repository_name is the repository where your package can be found, and it was probably installed from the first one in a list (see yum repolist)

So I know this is a somewhat old question - but it comes up on the google search for what I was looking for.

There is also a method to get a unique list of repos used:

# repoquery -ai | grep 'Repository  : ' | sort | uniq

This will list the repositories that have had packages installed from.