See https://github.com/LouisBrunner/valgrind-macos/.
Either try
brew tap LouisBrunner/valgrind
brew install --HEAD LouisBrunner/valgrind/valgrind
Or compile it from the source, if the above method doesn’t work. (But they should have fixed the above method just recently.)
git clone https://github.com/LouisBrunner/valgrind-macos.git
cd valgrind-macos
./autogen.sh
./configure --prefix=/where/you/want/it/installed --enable-only64bit
make
sudo make install
Enjoy!