For anyone looking for a solution for this on macOS Sierra 10.12 (or later, most likely): I fixed this by installing the command line tools:

xcode-select --install

After that, pip install psycopg2 should work.

If it doesn't, you could also try to link against brew's openssl:

env LDFLAGS="-I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib" pip install psycopg2

with openssl installed via brew. Note that the brew link openssl --force does not work anymore:

$ brew link openssl --force                                                                                 17.5s
Warning: Refusing to link: openssl
Linking keg-only openssl means you may end up linking against the insecure,
deprecated system OpenSSL while using the headers from Homebrew's openssl.
Instead, pass the full include/library paths to your compiler e.g.:
  -I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib

As @macho points out below if this still does not work, you might need to use the --no-cache option of pip, e.g.

env LDFLAGS="-I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib" pip --no-cache install psycopg2

Remember to adjust these paths accordingly should you for instance build on ARM/Apple M1 Macs (as homebrew is installed at /opt/homebrew/); command as follows:

env LDFLAGS="-I/opt/homebrew/opt/openssl/include -L/opt/homebrew/opt/openssl/lib" pip --no-cache install psycopg2

I had OpenSSL installed from brew (brew install openssl)

The following worked for me:

export LDFLAGS="-L/usr/local/opt/openssl/lib"
export CPPFLAGS="-I/usr/local/opt/openssl/include"
pip install psycopg2

Using brew link openssl is dangerous as it might mess up your system by symlinking Homebrew's OpenSSL headers while the actual libraries will remain the system-supplied ones, causing all kinds of issues. Homebrew actually warns you against this if you try (and other answers suggest linking won't solve the problem anymore anyway):

$ brew link openssl
Warning: Refusing to link: openssl
Linking keg-only openssl means you may end up linking against the insecure,
deprecated system OpenSSL while using the headers from Homebrew's openssl.
Instead, pass the full include/library paths to your compiler e.g.:
  -I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib

Following this advice here's the pip command you need to use:

$ pip install -r requirements.txt --global-option=build_ext --global-option="-I/usr/local/opt/openssl/include" --global-option="-L/usr/local/opt/openssl/lib"

For pipenv, I am not aware of any command-line attributes you can pass to it, however you can export the aforementioned paths as environment variables prior to running pipenv install:

$ export LDFLAGS="-L/usr/local/opt/openssl/lib" export CPPFLAGS="-I/usr/local/opt/openssl/include"
$ pipenv install

With MacOS Catalina 10.15.4, the following was the only command that worked for me:

env LDFLAGS="-I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib" pip install psycopg2

What worked for me was the hint provided in the command to link openssl,

$ brew link openssl
Warning: Refusing to link macOS-provided software: openssl
If you need to have openssl first in your PATH run:
  echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.zshrc

For compilers to find openssl you may need to set:
  export LDFLAGS="-L/usr/local/opt/openssl/lib"
  export CPPFLAGS="-I/usr/local/opt/openssl/include"

$ export LDFLAGS="-L/usr/local/opt/openssl/lib"
$ export CPPFLAGS="-I/usr/local/opt/openssl/include"

$ pip install psycopg2
Collecting psycopg2
  Using cached 
Installing collected packages: psycopg2
  Running setup.py install for psycopg2 ... done
Successfully installed psycopg2-2.8.2

On mojave I added these to the .bash_profile

export PATH="/usr/local/opt/openssl/bin:$PATH"
export LDFLAGS="-L/usr/local/opt/curl/lib -L/usr/local/opt/openssl/lib"
export CPPFLAGS="-I/usr/local/opt/curl/include -I/user/local/opt/openssl/include"

was then able to install psycopg 2.8.3 in a python 3.7.4 virtualenv.

This after reinstalling xcode and the command line tools.

All the answers above helped!

Solution for MacOS 12 Monterey - M1 Chipset:

xcode-select --install
brew install openssl
echo 'export PATH="/opt/homebrew/opt/openssl@3/bin:$PATH"' >> ~/.zshrc
echo 'export LIBRARY_PATH=$LIBRARY_PATH:/opt/homebrew/opt/openssl@3/lib/' >> ~/.zshrc

Solution for MacOS BigSur (May 2021)

I was having a similar issue. But just befor kicking my pc of the desk I found a solution, that worked for me on MacOS BigSur:

My OpenSSL installation was corrupted.

brew reinstall openssl

After the reinstallation is completed it will spit out 3 commands you need to execute:

The commands will look like this:

DONT JUST COPY

echo 'export PATH="/opt/homebrew/opt/openssl@1.1/bin:$PATH"' >> ~/.zshrc

export LDFLAGS="-L/opt/homebrew/opt/openssl@1.1/lib"
export CPPFLAGS="-I/opt/homebrew/opt/openssl@1.1/include"

This worked for me! There is no guarantee that this will work for you too.

brew reinstall might not be the best solution, but it worked and provided the necessary commands.

This's the problem of new macOs version, where pip cannot install cryptography. What fixed my problem is to provide the env to the install command:

brew install openssl
env LDFLAGS="-L$(brew --prefix openssl)/lib" CFLAGS="-I$(brew --prefix openssl)/include" <YOUR COMMAND HERE>

You can replace <YOUR COMMAND HERE> with pip install cryptography, or pip install <SOMETHING THAT REQUIRES cryptography> for example.

Credit to this article: Fixing macOS Sierra fatal error: 'openssl/opensslv.h' or 'openssl/aes.h' file not found

Using Fish, the following two commands solved this issue for me after installing OpenSSL with Homebrew.

set -gx LDFLAGS "-L/usr/local/opt/openssl/lib"
set -gx CPPFLAGS "-I/usr/local/opt/openssl/include"

Use brew info openssl to get up-to-date info.

export LIBRARY_PATH=$LIBRARY_PATH:/usr/local/opt/openssl/lib/

worked for me

If the export LIBRARY_PATH=$LIBRARY_PATH:/usr/local/opt/openssl/lib/ command doesn't change anything for you, then you may also need to specify openssl version in this path like

export LIBRARY_PATH=$LIBRARY_PATH:/usr/local/opt/openssl@1.1/lib/

Note the @1.1 after openssl.

If you:

• Are trying to install psycopg2 inside of a Python venv AND
• Are trying to install psycopg2 on Apple Silicon (new ARM CPUs)

Then, as IamAshay directed in their answer, we need to export some variables to point to our OpenSSL installation.

The install directory on Apple Silicon is apparently different ofrom Apple Intel machines. Regardless, to find where OpenSSL is installed (via homebrew) on your system, you can run

brew --prefix openssl

In my case, we have /opt/homebrew/opt/openssl@1.1. So this means that the variables we need to export would be

export LDFLAGS="-L/opt/homebrew/opt/openssl/lib"
export CPPFLAGS="-I/opt/homebrew/opt/openssl/include"

Then we can run

pip install psycopg2

Recently had this problem in High Sierra, having just installed Python 3.7 in a virtualenv.

The solution is to use a later version of psycopg2. Version 2.7.7 worked, where 2.7.1 did not.

Instead of installing psycopg2, install psycopg2-binary, from the same authors:

pip install psycopg2-binary

This is what the documentation says about this PyPI package:

You can [...] obtain a stand-alone package, not requiring a compiler or external libraries, by installing the psycopg2-binary package from PyPI:

$ pip install psycopg2-binary

The binary package is a practical choice for development and testing but in production it is advised to use the package built from sources.

I installed OpenSSL using MacPorts therefore directories are not like those of Brew.

sudo port install openssl

I found the directories by doing:

port contents openssl | grep lib
port contents openssl | grep include

Then I exported the variables:

export LDFLAGS="-L/opt/local/lib"
export CPPFLAGS="-I/opt/local/include/openssl"

You might also have to:

xcode-select --install

If using homebrew, it may not be the best idea to force link openssl. As some above answers outlined, the following worked for me inside a venv:

export LDFLAGS="-L/opt/homebrew/opt/openssl@1.1/lib"
export CPPFLAGS="-I/opt/homebrew/opt/openssl@1.1/include"
pip install psycopg2

I was having this issue on Mojave. Mojave does not create a /usr/include directory, which psycopg2 needs to install. This was not obvious. I found the solution here: How to update Xcode from command line, which references:

I had this same error and got it to resolve after I pip installed cython

Running PyCharm from conda environment, solved my issue using:

--> conda install psycopg2
The following packages will be UPDATED: ...

...
Proceed ([y]/n)? 
--> y
--> pip3 install psycopg2
Installing collected packages: psycopg2
Running setup.py install for psycopg2 ... done
Successfully installed psycopg2-2.8.4

'''

I know you are asking for development environment but if you are deploying on server say, Heroku. Just add below line in the requirements.txt of your project.

django-heroku==0.3.1

As this package itself will install the required packages like psycopg2 on server deployment. So let heroku server handle this.

In my case, openssl is located inside homebrew directory and suggested answers don't work.

Bad:

LDFLAGS="-I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib"