OkHttpCertPin.java

import com.datatheorem.android.trustkit.TrustKit;
import com.datatheorem.android.trustkit.config.PublicKeyPin;

import javax.inject.Inject;

import okhttp3.CertificatePinner;
import okhttp3.OkHttpClient;

public class OkHttpCertPin {
    private final TrustKit mTrustKit;
    @Inject
    public OkHttpCertPin(TrustKit trustKit)
    {
        mTrustKit = trustKit;
    }

    public OkHttpClient extend(OkHttpClient currentClient, String hostName) {
        CertificatePinner.Builder certificatePinnerBuilder = new CertificatePinner.Builder();
        for (PublicKeyPin key : mTrustKit.getConfiguration().getPolicyForHostname(hostName).getPublicKeyPins()) {
            certificatePinnerBuilder.add(hostName, "sha256/" + key.toString());
        }
        return currentClient.newBuilder().certificatePinner(certificatePinnerBuilder.build()).build();
    }
}

@cihadturhan I think RN fetch API uses XmlHttpRequest, haven't had a look at that, and axios does use it. In any case, this works for okhttp3 in android and the corresponding in iOS (I'm not much of an iOS developer).
@karthiganesan90 yes, it does not work anymore, not like that at least, depending on the version of RN you are using.
0.54 adds back this ability through OkHttpClientFactory. I did something like this:

OkHttpClientProvider.setOkHttpClientFactory(new CustomClientFactory());

then in your factory class, override method createNewNetworkModuleClient() and you'll be able to feed the client you need to the networking module, responsible for networking.

In order to match with @omerlh gist, here's the MainActivity.java for react native to use SSL Pinning on Android
Using RN 0.51 with Axios

package com.yourpackage;

import android.os.Bundle;

import com.facebook.react.ReactActivity;
import com.datatheorem.android.trustkit.TrustKit;
import com.facebook.react.modules.network.OkHttpClientProvider;
import okhttp3.OkHttpClient;

public class MainActivity extends ReactActivity {
    private String hostname = "your.server.com";
    private OkHttpClient currentClient;
    /**
     * Returns the name of the main component registered from JavaScript.
     * This is used to schedule rendering of the component.
     */
    @Override
    protected String getMainComponentName() {
        return "YourApp";
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TrustKit.initializeWithNetworkSecurityConfiguration(this, R.xml.network_security_config);
        currentClient = new OkHttpClient().newBuilder()
                                .sslSocketFactory(TrustKit.getInstance().getSSLSocketFactory(hostname), TrustKit.getInstance().getTrustManager(hostname))
                                .build();
        rebuildOkHtttp();
    }

    private void rebuildOkHtttp() {
        OkHttpCertPin certPin = new OkHttpCertPin(TrustKit.getInstance());
        OkHttpClient replacementClient = certPin.extend(currentClient, hostname);
        OkHttpClientProvider.replaceOkHttpClient(replacementClient);
    }
}