diff --git a/CHANGELOG.md b/CHANGELOG.md
index 26b1334c064..c38c3497544 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ We use *breaking* word for marking changes that are not backward compatible (rel
 
 ### Fixed
 
+- [#1955](https://github.com/thanos-io/thanos/pull/1955) Sidecar: enable command line flag to set http connection pool size `--prometheus.connection-pool-size`
 - [#1919](https://github.com/thanos-io/thanos/issues/1919) Compactor: Fixed potential data loss when uploading older blocks, or upload taking long time while compactor is
 running.
 - [#1937](https://github.com/thanos-io/thanos/pull/1937) Compactor: Improved synchronization of meta JSON files.
diff --git a/cmd/thanos/sidecar.go b/cmd/thanos/sidecar.go
index f6961e9766b..52161f32c02 100644
--- a/cmd/thanos/sidecar.go
+++ b/cmd/thanos/sidecar.go
@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"math"
+	"net/http"
 	"net/url"
 	"sync"
 	"time"
@@ -18,6 +19,7 @@ import (
 	"github.com/thanos-io/thanos/pkg/block/metadata"
 	"github.com/thanos-io/thanos/pkg/component"
 	"github.com/thanos-io/thanos/pkg/extflag"
+	"github.com/thanos-io/thanos/pkg/exthttp"
 	thanosmodel "github.com/thanos-io/thanos/pkg/model"
 	"github.com/thanos-io/thanos/pkg/objstore/client"
 	"github.com/thanos-io/thanos/pkg/prober"
@@ -30,6 +32,8 @@ import (
 	"github.com/thanos-io/thanos/pkg/store"
 	"github.com/thanos-io/thanos/pkg/store/storepb"
 	"github.com/thanos-io/thanos/pkg/tls"
+	"github.com/thanos-io/thanos/pkg/tracing"
+
 	"gopkg.in/alecthomas/kingpin.v2"
 )
 
@@ -45,6 +49,9 @@ func registerSidecar(m map[string]setupFunc, app *kingpin.Application) {
 	promReadyTimeout := cmd.Flag("prometheus.ready_timeout", "Maximum time to wait for the Prometheus instance to start up").
 		Default("10m").Duration()
 
+	connectionPoolSize := cmd.Flag("receive.connection-pool-size", "Controls the http MaxIdleConns. Default is 0, which is unlimited").Int()
+	connectionPoolSizePerHost := cmd.Flag("receive.connection-pool-size-per-host", "Controls the http MaxIdleConnsPerHost").Default("100").Int()
+
 	dataDir := cmd.Flag("tsdb.path", "Data directory of TSDB.").
 		Default("./data").String()
 
@@ -95,6 +102,8 @@ func registerSidecar(m map[string]setupFunc, app *kingpin.Application) {
 			*ignoreBlockSize,
 			component.Sidecar,
 			*minTime,
+			*connectionPoolSize,
+			*connectionPoolSizePerHost,
 		)
 	}
 }
@@ -120,6 +129,8 @@ func runSidecar(
 	ignoreBlockSize bool,
 	comp component.Component,
 	limitMinTime thanosmodel.TimeOrDurationValue,
+	connectionPoolSize int,
+	connectionPoolSizePerHost int,
 ) error {
 	var m = &promMetadata{
 		promURL: promURL,
@@ -243,8 +254,12 @@ func runSidecar(
 	}
 
 	{
-		promStore, err := store.NewPrometheusStore(
-			logger, nil, promURL, component.Sidecar, m.Labels, m.Timestamps)
+		t := exthttp.NewTransport()
+		t.MaxIdleConnsPerHost = connectionPoolSizePerHost
+		t.MaxIdleConns = connectionPoolSize
+		c := &http.Client{Transport: tracing.HTTPTripperware(logger, t)}
+
+		promStore, err := store.NewPrometheusStore(logger, c, promURL, component.Sidecar, m.Labels, m.Timestamps)
 		if err != nil {
 			return errors.Wrap(err, "create Prometheus store")
 		}
diff --git a/docs/components/sidecar.md b/docs/components/sidecar.md
index 4bdd8ced465..8823051eaf8 100644
--- a/docs/components/sidecar.md
+++ b/docs/components/sidecar.md
@@ -126,6 +126,9 @@ Flags:
       --prometheus.ready_timeout=10m
                                  Maximum time to wait for the Prometheus
                                  instance to start up
+      --prometheus.connection-pool-size=100
+                                 sets the MaxIdleConnsPerHost and MaxIdleConns
+                                 when connecting to prometheus
       --tsdb.path="./data"       Data directory of TSDB.
       --reloader.config-file=""  Config file watched by the reloader.
       --reloader.config-envsubst-file=""
diff --git a/pkg/exthttp/transport.go b/pkg/exthttp/transport.go
new file mode 100644
index 00000000000..92725c9b337
--- /dev/null
+++ b/pkg/exthttp/transport.go
@@ -0,0 +1,24 @@
+package exthttp
+
+import (
+	"net"
+	"net/http"
+	"time"
+)
+
+// NewTransport creates a new http.Transport with default settings.
+func NewTransport() *http.Transport {
+	return &http.Transport{
+		Proxy: http.ProxyFromEnvironment,
+		DialContext: (&net.Dialer{
+			Timeout:   30 * time.Second,
+			KeepAlive: 30 * time.Second,
+			DualStack: true,
+		}).DialContext,
+		ForceAttemptHTTP2:     true,
+		MaxIdleConns:          100,
+		IdleConnTimeout:       90 * time.Second,
+		TLSHandshakeTimeout:   10 * time.Second,
+		ExpectContinueTimeout: 1 * time.Second,
+	}
+}
diff --git a/pkg/store/prometheus.go b/pkg/store/prometheus.go
index 4012eceb9c8..41fa52e3e6d 100644
--- a/pkg/store/prometheus.go
+++ b/pkg/store/prometheus.go
@@ -27,6 +27,7 @@ import (
 	"github.com/prometheus/prometheus/storage/remote"
 	"github.com/prometheus/prometheus/tsdb/chunkenc"
 	"github.com/thanos-io/thanos/pkg/component"
+	"github.com/thanos-io/thanos/pkg/exthttp"
 	"github.com/thanos-io/thanos/pkg/runutil"
 	"github.com/thanos-io/thanos/pkg/store/storepb"
 	"github.com/thanos-io/thanos/pkg/tracing"
@@ -79,7 +80,7 @@ func NewPrometheusStore(
 	}
 	if client == nil {
 		client = &http.Client{
-			Transport: tracing.HTTPTripperware(logger, http.DefaultTransport),
+			Transport: tracing.HTTPTripperware(logger, exthttp.NewTransport()),
 		}
 	}
 	p := &PrometheusStore{
diff --git a/pkg/store/prometheus_test.go b/pkg/store/prometheus_test.go
index 5db3ee75cc9..bf6fee5cc76 100644
--- a/pkg/store/prometheus_test.go
+++ b/pkg/store/prometheus_test.go
@@ -14,6 +14,7 @@ import (
 	"github.com/prometheus/prometheus/prompb"
 	"github.com/prometheus/prometheus/tsdb"
 	"github.com/prometheus/prometheus/tsdb/chunkenc"
+
 	"github.com/thanos-io/thanos/pkg/component"
 	"github.com/thanos-io/thanos/pkg/store/storepb"
 	"github.com/thanos-io/thanos/pkg/testutil"
@@ -364,8 +365,7 @@ func TestPrometheusStore_Series_MatchExternalLabel_e2e(t *testing.T) {
 
 	proxy, err := NewPrometheusStore(nil, nil, u, component.Sidecar,
 		func() labels.Labels { return labels.FromStrings("region", "eu-west") },
-		func() (int64, int64) { return 0, math.MaxInt64 },
-	)
+		func() (int64, int64) { return 0, math.MaxInt64 })
 	testutil.Ok(t, err)
 	srv := newStoreSeriesServer(ctx)
 
@@ -410,8 +410,7 @@ func TestPrometheusStore_Info(t *testing.T) {
 
 	proxy, err := NewPrometheusStore(nil, nil, nil, component.Sidecar,
 		func() labels.Labels { return labels.FromStrings("region", "eu-west") },
-		func() (int64, int64) { return 123, 456 },
-	)
+		func() (int64, int64) { return 123, 456 })
 	testutil.Ok(t, err)
 
 	resp, err := proxy.Info(ctx, &storepb.InfoRequest{})
@@ -489,8 +488,7 @@ func TestPrometheusStore_Series_SplitSamplesIntoChunksWithMaxSizeOfUint16_e2e(t
 
 		proxy, err := NewPrometheusStore(nil, nil, u, component.Sidecar,
 			func() labels.Labels { return labels.FromStrings("region", "eu-west") },
-			func() (int64, int64) { return 0, math.MaxInt64 },
-		)
+			func() (int64, int64) { return 0, math.MaxInt64 })
 		testutil.Ok(t, err)
 
 		// We build chunks only for SAMPLES method. Make sure we ask for SAMPLES only.