diff --git a/.chloggen/starttime-fallback.yaml b/.chloggen/starttime-fallback.yaml
new file mode 100644
index 000000000000..adb5f64d6df7
--- /dev/null
+++ b/.chloggen/starttime-fallback.yaml
@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: prometheusreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add `receiver.prometheusreceiver.UseCollectorStartTimeFallback` featuregate for the start time metric adjuster to use the collector start time as an approximation of process start time as a fallback.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [36364]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []
diff --git a/receiver/prometheusreceiver/README.md b/receiver/prometheusreceiver/README.md
index cfa7472eb824..da1c93551f0b 100644
--- a/receiver/prometheusreceiver/README.md
+++ b/receiver/prometheusreceiver/README.md
@@ -67,7 +67,17 @@ prometheus --config.file=prom.yaml
 ```shell
 "--feature-gates=receiver.prometheusreceiver.UseCreatedMetric"
 ```
+- `receiver.prometheusreceiver.UseCollectorStartTimeFallback`:  enables using
+  the collector start time as the metric start time if the
+  process_start_time_seconds metric yields no result (for example if targets
+  expose no process_start_time_seconds metric). This is useful when the collector
+  start time is a good approximation of the process start time - for example in
+  serverless workloads when the collector is deployed as a sidecar. To enable it,
+  use the following feature gate option:
 
+```shell
+"--feature-gates=receiver.prometheusreceiver.UseCollectorStartTimeFallback"
+```
 - `receiver.prometheusreceiver.EnableNativeHistograms`: process and turn native histogram metrics into OpenTelemetry exponential histograms. For more details consult the [Prometheus native histograms](#prometheus-native-histograms) section.
 
 ```shell
diff --git a/receiver/prometheusreceiver/internal/starttimemetricadjuster.go b/receiver/prometheusreceiver/internal/starttimemetricadjuster.go
index 1b3eb51529f5..e740c891a495 100644
--- a/receiver/prometheusreceiver/internal/starttimemetricadjuster.go
+++ b/receiver/prometheusreceiver/internal/starttimemetricadjuster.go
@@ -6,7 +6,9 @@ package internal // import "github.com/open-telemetry/opentelemetry-collector-co
 import (
 	"errors"
 	"regexp"
+	"time"
 
+	"go.opentelemetry.io/collector/featuregate"
 	"go.opentelemetry.io/collector/pdata/pmetric"
 	"go.uber.org/zap"
 )
@@ -15,8 +17,27 @@ var (
 	errNoStartTimeMetrics             = errors.New("start_time metric is missing")
 	errNoDataPointsStartTimeMetric    = errors.New("start time metric with no data points")
 	errUnsupportedTypeStartTimeMetric = errors.New("unsupported data type for start time metric")
+
+	// approximateCollectorStartTime is the approximate start time of the
+	// collector. Used as a fallback start time for metrics that don't have a
+	// start time set (when the
+	// receiver.prometheusreceiver.UseCollectorStartTimeFallback feature gate is
+	// enabled).  Set when the component is initialized.
+	approximateCollectorStartTime time.Time
+)
+
+var useCollectorStartTimeFallbackGate = featuregate.GlobalRegistry().MustRegister(
+	"receiver.prometheusreceiver.UseCollectorStartTimeFallback",
+	featuregate.StageAlpha,
+	featuregate.WithRegisterDescription("When enabled, the Prometheus receiver's"+
+		" start time metric adjuster will fallback to using the collector start time"+
+		" when a start time is not available"),
 )
 
+func init() {
+	approximateCollectorStartTime = time.Now()
+}
+
 type startTimeMetricAdjuster struct {
 	startTimeMetricRegex *regexp.Regexp
 	logger               *zap.Logger
@@ -33,7 +54,11 @@ func NewStartTimeMetricAdjuster(logger *zap.Logger, startTimeMetricRegex *regexp
 func (stma *startTimeMetricAdjuster) AdjustMetrics(metrics pmetric.Metrics) error {
 	startTime, err := stma.getStartTime(metrics)
 	if err != nil {
-		return err
+		if !useCollectorStartTimeFallbackGate.IsEnabled() {
+			return err
+		}
+		stma.logger.Info("Couldn't get start time for metrics. Using fallback start time.", zap.Error(err), zap.Time("fallback_start_time", approximateCollectorStartTime))
+		startTime = float64(approximateCollectorStartTime.Unix())
 	}
 
 	startTimeTs := timestampFromFloat64(startTime)
diff --git a/receiver/prometheusreceiver/internal/starttimemetricadjuster_test.go b/receiver/prometheusreceiver/internal/starttimemetricadjuster_test.go
index 84bdc2756ed5..4bfc6abc2237 100644
--- a/receiver/prometheusreceiver/internal/starttimemetricadjuster_test.go
+++ b/receiver/prometheusreceiver/internal/starttimemetricadjuster_test.go
@@ -6,11 +6,14 @@ package internal
 import (
 	"regexp"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"go.opentelemetry.io/collector/pdata/pcommon"
 	"go.opentelemetry.io/collector/pdata/pmetric"
 	"go.uber.org/zap"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/internal/common/testutil"
 )
 
 func TestStartTimeMetricMatch(t *testing.T) {
@@ -154,3 +157,96 @@ func TestStartTimeMetricMatch(t *testing.T) {
 		})
 	}
 }
+
+func TestStartTimeMetricFallback(t *testing.T) {
+	const startTime = pcommon.Timestamp(123 * 1e9)
+	const currentTime = pcommon.Timestamp(126 * 1e9)
+	mockStartTime := time.Now().Add(-10 * time.Hour)
+	mockStartTimeSeconds := float64(mockStartTime.Unix())
+	processStartTime := mockStartTime.Add(-10 * time.Hour)
+	processStartTimeSeconds := float64(processStartTime.Unix())
+
+	tests := []struct {
+		name                 string
+		inputs               pmetric.Metrics
+		startTimeMetricRegex *regexp.Regexp
+		expectedStartTime    pcommon.Timestamp
+		expectedErr          error
+	}{
+		{
+			name: "regexp_match_metric_no_fallback",
+			inputs: metrics(
+				sumMetric("test_sum_metric", doublePoint(nil, startTime, currentTime, 16)),
+				histogramMetric("test_histogram_metric", histogramPoint(nil, startTime, currentTime, []float64{1, 2}, []uint64{2, 3, 4})),
+				summaryMetric("test_summary_metric", summaryPoint(nil, startTime, currentTime, 10, 100, []float64{10, 50, 90}, []float64{9, 15, 48})),
+				sumMetric("example_process_start_time_seconds", doublePoint(nil, startTime, currentTime, processStartTimeSeconds)),
+				sumMetric("process_start_time_seconds", doublePoint(nil, startTime, currentTime, processStartTimeSeconds)),
+				exponentialHistogramMetric("test_exponential_histogram_metric", exponentialHistogramPointSimplified(nil, startTime, currentTime, 3, 1, -5, 3)),
+			),
+			startTimeMetricRegex: regexp.MustCompile("^.*_process_start_time_seconds$"),
+			expectedStartTime:    timestampFromFloat64(processStartTimeSeconds),
+		},
+		{
+			name: "regexp_no_regex_match_metric_fallback",
+			inputs: metrics(
+				sumMetric("test_sum_metric", doublePoint(nil, startTime, currentTime, 16)),
+				histogramMetric("test_histogram_metric", histogramPoint(nil, startTime, currentTime, []float64{1, 2}, []uint64{2, 3, 4})),
+				summaryMetric("test_summary_metric", summaryPoint(nil, startTime, currentTime, 10, 100, []float64{10, 50, 90}, []float64{9, 15, 48})),
+			),
+			startTimeMetricRegex: regexp.MustCompile("^.*_process_start_time_seconds$"),
+			expectedStartTime:    timestampFromFloat64(mockStartTimeSeconds),
+		},
+		{
+			name: "match_no_match_metric_fallback",
+			inputs: metrics(
+				sumMetric("test_sum_metric", doublePoint(nil, startTime, currentTime, 16)),
+				histogramMetric("test_histogram_metric", histogramPoint(nil, startTime, currentTime, []float64{1, 2}, []uint64{2, 3, 4})),
+				summaryMetric("test_summary_metric", summaryPoint(nil, startTime, currentTime, 10, 100, []float64{10, 50, 90}, []float64{9, 15, 48})),
+			),
+			expectedStartTime: timestampFromFloat64(mockStartTimeSeconds),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			testutil.SetFeatureGateForTest(t, useCollectorStartTimeFallbackGate, true)
+			stma := NewStartTimeMetricAdjuster(zap.NewNop(), tt.startTimeMetricRegex)
+			if tt.expectedErr != nil {
+				assert.ErrorIs(t, stma.AdjustMetrics(tt.inputs), tt.expectedErr)
+				return
+			}
+
+			// To test that the adjuster is using the fallback correctly, override the fallback time to use
+			// directly.
+			approximateCollectorStartTime = mockStartTime
+
+			assert.NoError(t, stma.AdjustMetrics(tt.inputs))
+			for i := 0; i < tt.inputs.ResourceMetrics().Len(); i++ {
+				rm := tt.inputs.ResourceMetrics().At(i)
+				for j := 0; j < rm.ScopeMetrics().Len(); j++ {
+					ilm := rm.ScopeMetrics().At(j)
+					for k := 0; k < ilm.Metrics().Len(); k++ {
+						metric := ilm.Metrics().At(k)
+						switch metric.Type() {
+						case pmetric.MetricTypeSum:
+							dps := metric.Sum().DataPoints()
+							for l := 0; l < dps.Len(); l++ {
+								assert.Equal(t, tt.expectedStartTime, dps.At(l).StartTimestamp())
+							}
+						case pmetric.MetricTypeSummary:
+							dps := metric.Summary().DataPoints()
+							for l := 0; l < dps.Len(); l++ {
+								assert.Equal(t, tt.expectedStartTime, dps.At(l).StartTimestamp())
+							}
+						case pmetric.MetricTypeHistogram:
+							dps := metric.Histogram().DataPoints()
+							for l := 0; l < dps.Len(); l++ {
+								assert.Equal(t, tt.expectedStartTime, dps.At(l).StartTimestamp())
+							}
+						}
+					}
+				}
+			}
+		})
+	}
+}