opencv is not compatible with python 3. I had to install opencv3 for python 3. The marked answer in how could we install opencv on anaconda? explains how to install opencv(3) for anaconda:

Run the following command:

conda install -c https://conda.binstar.org/menpo opencv

I realized that opencv3 is also available now, run the following command:

conda install -c https://conda.binstar.org/menpo opencv3

Edit on Aug 18, 2016: You may like to add the "menpo" channel permanently by:

conda config --add channels menpo

And then opencv can be installed by:

conda install opencv (or opencv3)

Edit on Aug 14, 2017: "clinicalgraphics" channel provides relatively newer vtk version for very recent python3

conda install -c clinicalgraphics vtk

Edit on April 16, 2020 (based on @AMC's comment): OpenCV can be installed through conda-forge (details see here)

conda install -c conda-forge opencv

Just open your command prompt with run as administrator. And try,

conda install -c conda-forge opencv

First do run these commands inside Terminal/CMD:

conda update anaconda-navigator  
conda update navigator-updater  

Then the issue for the instruction below will be resolved

For windows if you have anaconda installed, you can simply do

pip install opencv-python

or

conda install -c https://conda.binstar.org/menpo opencv

if you are on linux you can do :

pip install opencv-python

or

conda install opencv 

Link1 Link2

For python3.5+ check these links : Link3 , Link4

Update:
if you use anaconda, you may simply use this as well (and hence don't need to add menpo channel):

conda install -c conda-forge opencv