Android Picasso, How to add authentication headers?

Try configuring an OkHttp3 client with authenticator, depending on your scheme and situation:

OkHttpClient client = new OkHttpClient.Builder()
        .addInterceptor(new Interceptor() {
            @Override
            public Response intercept(Chain chain) throws IOException {
                Request newRequest = chain.request().newBuilder()
                        .addHeader("Authorization", "bearer <accesstoeken>")
                        .build();
                return chain.proceed(newRequest);
            }
        })
        .build();

Picasso picasso = new Picasso.Builder(context)
        .downloader(new OkHttp3Downloader(client))
        .build();

Now use it as:

picasso.load(imgUrl).into(holder.mImgProfileView);